You have an opportunity to rebuild with features that will give you a TT superior to most other TTs built today.
Insulation is one big thing. Spray-in definitely THE best thing to use. Pay attention to insulating under the floor as best as possible. You could give it a heated and enclosed underbelly but heating off the furnace is very inefficient. Better to use heating blankets under the tanks (if camping in cold weather) and keep the lines within the interior heated space.
Stay with wood framing. You'd have a lot of studs which would transfer more heat and cold than wood. I would make the exterior studs a bit wider to allow for thicker insulation. Never enough insulation in a TT... Instead of jalousie windows, what about frameless? They sure look nice, don't let rain in and IMO still allow adequate ventilation (fine for us in 105F heat). Install at least one larger window and make it high enough above the floor to see out when standing. (I hate the typical small low down windows that make it dark inside and difficult to gaze out.)
For ceiling height, what about building an arched ceiling like some new TTs have? Ours has it and it's great. You could make your own curved rafters by laminating some plywood together. Might be able to get some made up too. Not sure if you could build them as open web trusses like they do with aluminum. Maybe consult a structural engineer.
I don't think the frame will be an issue unless you plan to add much weight. I've read some say that older frames are stronger than newer ones. Look for stress cracks in the I-beams around the spring hangers. Maybe add gussets above the spring hangers. To strengthen the frame, you *could* potentially weld a 1/8" flitch plate to the exterior sides of the I-beams, continuous from maybe 2-3' ahead of and to behind the spring hangers. Consult a frame shop on how to weld this. Yes, check the A-frame to I-beam welds - maybe go over them with new welds. If re-building from frame up, you could strengthen the frame as needed to support a rack at the rear.
Assess the existing subfloor to reduce bounce/flex. Are joists close enough together and what thickness is existing subfloor? 5/8" plywood would be a good choice with joists 16" OC.
Replace springs and install Dexter EZ-flex equalizers and wet bolts. Check holes in spring hangers for ovalizing. Install shocks!! One of the best things we've done to our TT.
Pressure treat plywood and lumber will not work in an RV.
If you want to use electric heat, one option is a permanent recessed kickspace heater under the range (if you have one) like I did in pic below. Same width as range which makes it easy. No more cube heater and cord laying around on the floor.
Lots of things you can improve upon. Too many to list here. :)
Good luck and have fun!
