Those are some good questions... Experience is, alas, the only way you can, yourself, know,, and of course since you have yet to go to the school of hard knocks,,, You don't have that.
Your best bet, for avoiding an advanced course at said school, is to cultivate a relationship with someone who has been there and learn from him... I can tell you a few thigns
Look at the rig.. I mean LOOK at it, walk through it,, If you see signs of water in the walls (Soft spots, bubbling of wall paper, de-lamination of the walls and that kind of thing) ... Thank the seller for his time.. and show him your posterior walking away... SWIFTLY!.
Likewise if in walking around the floor seems kind of .. oh,, bouncy (like you are in a moon walk) Walk out.. QUICKLY and look elsewhere.
Obvious signs of damage and repair.. You may wish to move on.
But beyond that.. Get a friend to go with you and look it over
Next.... If you believe a salesman.. A Chevy LUV pickup can pull a 20,000 pound 5th wheel like it was your child's little red wagon.
Of course..... Monty Python had a skit about that (Very funny) only it was a compact care and a large boat...
How much your Dodge Ram can safely tow.. I don't know.. DODGE, however, does.. Go to their website, or break out your owner's manual, and find out.. Do not push it.. If the truck can tow, way 12,000 pounds.. and the trailer weighs 10,000 pounds.. I'd look for something lighter. Figure you will add 1,000 lbs minmum to the trailer before you tow it.
Home is where I park it!