Well, the adventure begins.
To remove the belly pan you'll need a drill and several 1/8" bits. Drill out the center of the rivets. At least that's the way I did it and found it easier than trying to shave off the rivet heads. Once you've got the pan removed you'll need to get rid of all the pink insulation and whatever was living up there. Then you can inspect the frame, which is the weak part of the whole set up. If you're lucky, there'll be a few rust spots and maybe an outrigger that needs to be braced up. If that's the case, clean off the rust, brace the outrigger, and use a product like POR, or something by Rustoleum or whatever to kill the rust, then coat it with a good rust inhibiting paint or pickup bed liner (your choice, lots of different opinions). If you've got frame members rusted though then you also probably have rotted sub floor. Fortunately I have no experience with that.
If the axle was replaced with a new one (not a used one), then that's a good thing - leave it alone. Do check the brakes and bearings.
You should tow the trailer in as level a position as reasonable. Buy a hitch with the proper drop. Check the tires, they should be no more than 4 years old, 5 is pushing it.
Take care of the structural stuff first so it's safe to tow, then move on to plumbing and electrical.