RV commodes are all over the board, so this essay is about replacing a toilet that used 4 bolts to hold it down, with a toilet that uses just two bolts. In our case, both toilets were ceramic and quality, but the new toilet has an elongated seat; something the DW has gotten used to.
1. First, shut off water and disconnect the supply line.
2. Remove decorative caps and un-do the 4 nuts, then pull the old commode straight up until it clears the closet bolts. Put it outside.
3. Remove caulk or other junk from the flange after removing all four closet bolts. (photo 1)
4. Note that the flange is screwed to the floor with as many as eight screws. Remove them and try to twist the flange. Your goal is to get two closet bolt insert at the 3-9 position (OEM 4-bolts are more a 10-2 and 8-4, so they must be aligned in order for the new toilet to fit)
In our case, the flange was mated to the down-pipe, and wouldn't move...
(photo 2 shows washers where the holes need to be)
5. Two options exist at this point. Either install an overlay flange like Sealand Mounting Adapter Kit - 2/4 Bolt Conversion - Bone-pplmotorhomes.com
or drill holes in the old flange where the screws need to be.
That outer ring is needed because the new flange for RV use is thick enough that your new toilet will rock if it doesn't have that outer ring to sit on.
You can also go to a hardware store or big box and get a stainless flange overlay that doesn't require you to use the outer ring. For about $5, this flange fits outside the sealing washer, so it doesn't raise the commode (see photos) (photo 3 shows overlay flange and 4 shows how it fits outside of the toilet's rubber gasket)
NOTE: that slot with the boxey opening is where the closet bolts will go. Do NOT screw it down with the boxey openings at 3-9; you want the middle of the slots to be at that position.
Part 2 involves securing the commode and connecting the water line.