If you are gutting (IE removing wall and ceiling paneling) SCRAP the existing electrical system, both 12V AND 120V wiring, start from scratch and do it right the first time with heavier ga wire in the 12V side (mine had variety of wire sizes from 16 ga to 14 ga and bunches of splices) and use 12ga wire on the 120 side instead of 14 ga wire..
The wiring that RV manufacturers tends to be, well, substandard, IE junk.
I have found rats nests of 12V wiring with HIDDEN splices IN THE WALLS!!!
One rats nest included not only 12V wiring but also intertwined with 120V wiring and the 12V wiring had overheated at one point enough to melt insulation of all the wires into one massive glob with a few 12V circuits shorted to ground.. The PO of the trailer couldn't figure out why the bathroom lights and fan kept blowing fuses and must have upsized the fuses.. Paneling had char marks to prove something had gone wrong..
A new furnace is going to be a HUGE cost, they are insanely expensive, I was lucky to pick up a "scratch and dent" furnace for half price at one of the RV surplus stores in Elkhart (RV capitol of the world).. Even at half cost it was about $300 with shipping..
Blueprints of 1970s trailers, yeah, that IS a "rookie" mistake, not going to happen, most RV trailer builders do not release blueprints nor do they keep very old documentation.
You might be able to find a sales brochure with the layouts of their models for that year if you dig around the Internet.
I did manage to get a "owners manual" once but that was a 1980s trailer and the manufacturer told me that one year older and they would not have had it, many businesses scrap old documents 10 yrs after the model has been sunsetted (IE discontinued) or if they get bought out..
I think Winnebago MIGHT be the rare exception to that general rule and some motorhome manufacturers also.. But common garden variety travel trailers, not so much.
