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I would start by downloading the "buyer's checklist" and going through a complete inspection. This will help identify the areas that need to immediate attention.
The next question is, do you want to camp in it as soon as possible, or do you intend to do some major refurb before hand?
If you want to camp ASAP, then fix the things you found wrong during the inspection, and go for it.
If you intend to do a major overhaul anyway, then start at the "foundation:"
- verify the condition of the floor and rear end (70's trailers are notorious for floor rot and rear end separation)
- replace the tires, if needed,
- determine whether the axle needs to be replaced (answer is yes, if it the original axle)
I suggest starting with the floor because the floor rot/rear end separation decision is the point at which folks usually have to decide whether to drop the bellypan, remove the interior, and replace the entire floor or just sections of it. If you end up doing a shell-off, then the axles, tires, and frame all can get dealt with after that decision is made.