In the trade, this double paned glass is known as a unit. Several trade names are used to identify the manufacture of them. If your unit has a metal spacer (that is the aluminum separator) between the lites (pieces) of glass, you should see the manufacturer's trade name, stamped somewhere on the inside edge of this 1/4" (usually) spacer bar. At the same time, if you look closely at the perimeter of the glass (usually about 1" in), you will see that there is a sandblasted or acid etched name of the GLASS manufacturer. That name tells you that the glass is tempered safety glass. If you look at your windshield, lower left or right corner, you will see a similar marking. This indicates who manufactured the windshield and that it is a laminated safety glass. Side glass and rear glass in your automobile has similar markings, but this glass is tempered safety glass. These lites of tempered glass, were originally annealed glass (regular glass), cut to required size and then tempered to create safety glass. Tempered glass can not be recut.
OK. now that you are somewhat educated on units and glass, we can now discuss the manufacture of units.
The first unit commercially manufactured was the Thermopane. It used a lead spacer that was fused to the inside of the glass edge. These were produced as far back as the 1940s. Believe it or not, some of these still exist today, performing as well as they did when first manufactured. Then came the Anderson Window, All Glass unit. It literally was one lite of glass that was heated and folded over and edges were rolled together to create a air and water seal. They actually tested these finished units, by floating them in water, to insure that the all glass unit was sealed. Although both of these types of units were better than any produced today,the cost of manufacturing each was prohibitive. So in the very late sixties to early seventies, they started manufacturing units by laying one lite of glass on a flat surface, then laying an assembled hollow rolled aluminum spacer bar, that usually had one length filled with a desiccant to absorb moisture when the second lite of glass was placed onto the spacer and then sealed with either a polysulphide or polyurethane sealant. The seam of the rolled spacer was centered, facing inside of the unit, to allow encapsulated air, containing moisture, to enter into the hollow of the spacer, allowing the desiccant to do it's job. Little tricks of the trade included air applied sealant with special applicator tools, cleaning the inside surface with a glass washer, stacking units of same size as high as two feet or so. Visualize glass, spacer, glass, glass, spacer, glass, glass, spacer glass etc., forming a stack of unsealed units. They would then apply the sealant using a trowel to squeeze the sealant onto the slightly set in spacer bar and onto the glass edges. This would be allowed to cure for several hours, perhaps over night or the modern manufacturers ran the stack through a slow moving oven to cure the "Block of Black Sealant" that was applied to all 4 sides of the unit. After cure, the individual units would be separated by means of a utility knive. The units would then be cleaned of excess sealant on usually just the top and bottom lites of glass, in the stack. At this point, Argon or other gas CAN be introduced. Most units are manufactured without gas as it The Argon or similar gas is used to increase the energy efficiency of the unit. Truth be known, this gas will only last up to 2 years, inside the unit as all sealants, including silicone, will allow the gas to permeate through the seal. Holes are drilled through the sealant in the opposite corners of the unit, one to allow the gas to be pumped in while the other is to allow the trapped air to escape. The corners are then resealed with the same sealant that the unit was previously sealed with. After a brief cure the Units are now ready for shipping or in house glazing, depending on nature of the manufacture ie: some window manufacturers also make their own units while some unit manufacturers, produce for all trades.
OK - Glass - Units now covered.
REPAIRING SEAL FAILURE UNITS -
Yes, I have done it. Was it a great job? No. It can be a good job, if you are willing to accept a less than perfect unit or maybe even a unit that no longer holds water or moisture. It will never be perfect. You might achieve on a performance scale of 1-10, maybe a 5 or 6. On a perfectly clear vision, you might achieve a 5.
So you just removed the old unit from the frame - cutting the glazing sealant or removing the rubber gasket from around the unit. You may have noticed small pieces of vinyl or rubber under the bottom edge of the unit at 1/4 points (these are called setting blocks or shims) and possibly additional pieces at 1/2 points on both sides (these are called side shims). They are placed there to prevent the unit from moving side to side, against the metal frame. The bottom setting blocks are used to raise the unit up off the metal frame to allow any water that may flow down the glass face, settling under the unit, to drain away. They are also used to cushion the unit in the frame.
If the unit is set into a rubber gasket, the use of shims or setting blocks is not required.
OK, you now have the old unit in hand and it is looking pretty grungy, filled with moisture and it is quite stained. Lay the unit on a flat surface, covered by rug or towel. Using a utility knife, carefully cut the sealant, separating both lites of glass from the aluminum spacer bar. Trim off as much of the black sealant as possible from the exposed edges of the spacer bar, being careful not to bend it. You might be able to still hear the desiccant in the hollow spacer bar by shaking it. This desiccant is dead ! It did it's job as long as it could but was unable to absorb any more moisture that had permeated the sealant. Result? Seal Failure. You may be able to empty the expired desiccant from the spacer bar by gently opening the seam and replacing the desiccant (if you have a friendly unit manufacturer in your are who will give you a bit. Keep it in an air tight container until you are ready to use it.
Now, you have the two tempered pieces of glass in hand and are ready to clean them --- Windex??? NO ! It will leave a film on your glass, even though you make think it hasn't. No matter what you use to try and clean it, you're glass is permanently water stained and will never be like new.
As the original unit was manufactured probably 10 years or more ago, we can assume your RV is also that old and probably is starting to show it's age.
You can try cleaning the glass with vinegar and water, using a good deal of elbow grease. You might be content with how your glass looks after cleaning and you say "Good Enough". So be it. You can try using rubbing alcohol to assist in removing the stains as well. Although it is not recommended, I have used 'Easy Off Oven Cleaner' to remove real stubborn stains. It usually works, but excessive use of this caustic product will eventually pit your glass, if used repeatedly. I have used it to remove the black stains on glass that have been left behind from screens on windows and patio doors. I repeat -- do not use this product as a normal cleaner or use it excessively. No water which method you attempt to clean your glass, rinse well with plain water and then do it again and lastly, do it again !!! Get the cleaner off the surfaces !!! Allow glass to dry completely before starting the reassemble.
The most readily available sealant is silicone. If you are able to buy a neutral cure silicone, (Non acetic), the better for your seal. If you are able to purchase a gunnable urethane, that will also work. I have used acetic silicone for this application, but feel the neutral cure silicone is superior for the longevity of the unit. Lay the first lite of glass on your rug, place the spacer bar on top of this glass and then add your second lite of glass on top of the spacer bar. Square your unit to insure glass is positioned the same, top and bottom lites. The spacer should be evenly set in place equally spaced all around. If you have rubber tipped clamps, they should be used to hold the two lites of glass and spacer together. If you don't have clamps, have someone hold the top glass down, to insure that it does not move when you are applying the sealant. Sealant can now be evenly applied to the spacer and glass edge. Insure that the sealant FULLY covers the spacer bar to a depth of 1/8" +. A thin popsicle stick can be used to trowel your sealant to an even thickness. The void between the spacer and the glass edge need not necessarily to completely filled with sealant as long as sealant is extended beyond the spacer, on the the glass. If you want to fill the void, go ahead, 'knock yourself out', it will not harm the unit, it will just take longer for the sealant to cure. A full 100% cure usually takes up to 7 days, however, your unit should be able to be moved and excess sealant cleaned off your finished product and will be usable to be replaced in your frame.
You are now a glass and sealant expert
and your unit should perform as intended for several years, if you have followed all of the above.
You are now finished --- and so am I.
Sorry to go into so much detail but I just had to address the advice of "Well Meaning" Helpers.
As an added note: I have been in the glass and sealant business for over 50 years.