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By design, '69 and later trailers are particularly prone to rear floor rot, and a condition known as (the dreaded) rear-end separation. This happens when the wooden subfloor (which is sandwiched between the shell and the frame) rots completely away, allowing the shell and frame to move semi-independently of one another.
An easy way to check for rear end separation is to step up on the rear bumper and bounce lightly up and down while watching the rear of the trailer where the shell and frame meet. The two should move together--if you see them separating, then you are looking at replacing at least the rear most sheet of subfloor, probably doing repairs to the rear portion of the frame that is rotting away, and after you say to yourself a few times "well, as long as I am here..." you will be three years into a shell-off restoration. By the way, in order to replace that rear-most sheet of plywood, you will have to remove the entire bathroom, which is made of 46 year old plastic that will crumble as you try to take it apart.
So manage your expectations. The trailer is 46 years old, and you already know it has floor rot. It is very likely a candidate for a full rebuild. The $750 they are asking for the trailer (which is pretty cheap) will be long forgotten as you start replacing axles, appliances, etc..
Go to the "Portal" tab, and look for the buyer's inspection checklist. This will help you to identify all of the possible issues, and there will be a lot of them. This isn't about chewing the seller down on the price, as it is already little more than scrap value, it is about understanding how far gone the trailer is, and deciding if you are ready for that much of a project. Also have a look at some of the "full monty" rennovation threads. Most of these started out with a trailer that someone thought they would be camping in right away, only to discover rotting floors.