If the trailer is old it most likely has some structural damage do to water leaks. There tough to repair structural damage especially if they are used structural foam panel construction. I new this going into my renovation because of past experience. So I purchased a trailer destined for salvage. the trailer was only 8 years old. Basically I bought an RV trailer frame, but I had all the appliances, converter, holding tanks, windows etc. I demoed the trailer down to the frame and started over. The advantages for me were many. The trailer has no water leaks what so ever. I gained over a foot in length inside that allowed for 2 single beds, one on each side of the trailer with an isle almost the entire length of the trailer. The beds are easy to make and the mattresses are lite. There is a ton of very accessible storage under each bed. I kept the same floor plan except for the beds. I was able to rebuild things in a logical manner. By this I mean I was able to run electric and plumbing in a way that was not only more accessible but out of the way. this gained me even more storage under the frig, under the stove, beside the converter, above the furnace and under the sinks. So much so that we eliminated all the over head cabinets except over the kitchen sink. This really makes the 16 ft. x 7 ft. trailer size seam roomy and not so claustrophobic. It also eliminated hitting heads on cabinets over the beds and table. Things like the water pump, water heater, wiring connections are all accessible behind cabinet doors and aren't buried under wires or plumbing or built in behind panels that are not easily removed. I also dropped 300lbs. in trailer weight, and have real aluminum studs in the walls and around all the accesses, window and door openings. It's not a high end luxury trailer, but it does not look home built, it's lite, roomy, comfortable, dry and easy to work on if need be. And I'm a long way off the price of a new trailer that probably started leaking after being in it's first rain storm. Hope this is an inspiration. It was a fun project and I would do it again. Good luck with yours!
Tom