I haven't read thru this entire thread but I'll weight in, assuming you haven't already purchased something.
I saw
@Zorba mentioned leaks. These are the absolute killer of RVs. when you look at one, go over the entire ceiling pressing up lightly. Be sure to open the cabinets and do this inside the cabinets as well. if you feel any soft spots, it is, or has, leaked. For the price point you are looking at, I wouldn't want it to have any evidence of leaks whatsoever.
Towing capacity of your F150 is a concern. As far as safety, with something about the size you pointed to, I wouldn't worry about being safe assuming the truck and trailer brakes and everything are in working order and you have a hitch rated for the capacity. This doesn't mean you won't be doing 25mph up hills tho!
I have only had 2 older RVs, both from the 80s. My first one was basically the Breaking Bad RV. It needed a complete overhaul and I swapped out just about everything and had to fix frozen water pipes. I got it for free, and the joke (actually the truth) was that I paid too much.
My second and current one is an 87 and is in much better shape. I just re-sealed the roof. I've replaced the refrigerator and AC unit on it, and have done a bunch of other stuff. Both of mine were motorhomes, not tow behinds.
All that is to say that yeah, you will be constantly working on it. Before you buy, check all the systems. The fridge takes many hours to cool off, so be sure to tell them you want to see the fridge in action before you leave to go see it. Bring some thermometers to check the temps, and its best to see em when the temperature is hot as these absorption fridges are finicky, and expensive to fix or replace.
be sure the water system holds pressure. The water pump will shut off and stay off if it does.
Check the water heater and be sure it isn't bypassed when you are checking it. Many RVs have bypasses on the water heater plumbing so if you winterize with antifreeze you don't need the extra 6 or 8 gallons to fill it up as well. you will see valves near the water heater inside the rig. how they work will be obvious.
Step firmly on the floors to check for soft spots.
You could do a propane leak test, but that requires a monometer and a place to hook it up. Something of this age will probably be ok, but it is something you will want to do either before or after you purchase it. Look up how to do it. Also, if not equipped, get a propane/CO detector.
You might also want to check the age of the house batteries, and the age of the propane tanks. I think the propane tanks only hold certification for 10 years, which afterwards they need to be recertified or replaced or many places won't refill em.
If the RV has a ladder climb up and physically inspect the roof. I wouldn't walk on the roof, and the owners likely won't want you too, but just inspect it for any obvious damage. I don't walk on mine unless I absolutely have to. even then, I try to put down wood to spread the load. If you don't, you can feel for the supports and try to just walk where they are.
Obviously look underneath for any rust, but i think where you are, that's unlikely.
Check the condition and age of the tires.
I would jack the rig up and spin the wheels to check the condition of the wheel bearings.
That's about all I can think of at the moment, good luck, They are lots of fun, but yeah, they need constant maintenance.
