Patrick,
First, this respondent is the current owner of a 1973 GMC and was the owner of a 1979 Cayo/Dodge. Please take what I suggest very seriously.
This may sound stupid, but you have a house on a truck. The truck part is easy to deal with because you can find people that know about even one that is that old and there are books*.
The house is the hard part. To this end, start a notebook today. It should be part truck and part house. Collect the manufacturer and part number of everything you can right now. Put that all in the notebook. If you need documentation for the installed appliances, do a web search. I would bet that most can be found and probably many at the
GMC Manuals website.
If it truly is all structure aluminum and fiberglass, then you have a chance to keep this a very long time. You have to close off the leaks as soon as you can find them. Such leaks are usually visible from the outside, but it will take some looking. I have found that the marine caulking materials are the best. The vibration is more than what any household material can accommodate.
Start now at getting used to doing the basic maintenance that the truck requires so it does not give out on you. That will not be difficult. But, something to put in you schedule early is that all the rubber is suspect. It is just too old. I suspect that it has not had a heat problem, but age alone will get it. So, count on replacing all the rubber. The tires are a given. (If it has 16.5 rims, hunt up 16s as those old sizes are getting hard to find.) When I say all rubber parts, I mean all. Belts, Brake hoses, Coolant hoses, Fuel lines and door seals. Fortunately, all that stuff is work that can be done by a technician with a rating only slightly above "Yellow Canary". Do not try to salvage the coolant or brake fluid as they are also probably junk. Coolant usually is only good for about three years before the anti-corrosives are depleted and brake fluid absorbs water and so is only good for about five years.
Oh, an 318 LA is a really solid motor, just keep plug wires 5&7 apart so it does not cross-fire and The valve seats tend to wear with no-lead fuels. If that is an exhaust leak, start soaking the bolts in ATF/Acetone now or some good penetrent and maybe you can unscrew it.
*I think I saw a chassis book in my shop. If I can find it again, it is yours for shipping.
Good luck guy.
Matt
