Hello FootballMom. You might have a bigger problem than you know if somebody else said there's a leak. Was this a Michigan trailer? A lot can happen between 1968
and the present. Ignoring winterizing for one season only could have damaged the original copper pipes -- a hard freeze could irreversibly damage copper pipes.
Most piping runs on top of the floor. Looking under cabinets or removing a bed platform (1/4" plywood) may give you access to inspect. Look for split or swollen pipes or sweated junctions. Does anything leak visibly? Shower mixer valves are particularly susceptible to frost damage; this may involve more disassembly of shower & inner skin than you anticipated. Traps under sinks also deserve a look. Find your water pump and inspect. Many of us have been there.
You can get water into the system one of two ways. One is to fill up the fresh water tank, turn on the pump and see what works. The second is to look for a hose attachment (near the rear bumper on the roadside?). Airstreams before 1973 in original condition do not have a gray water tank; they'd just drain into a bucket under the trailer ... or a blueboy
Your hot water heater could have operational issues. That's a separate question. There may be three valves near the water heater (anybody? did they do this in the late 60s?). Put them in the 'Winter' position to bypass the water heater.
Hopefully your black tank is empty and doesn't have a baked mud brick inside. The black tank would be directly under the toilet. With a strong light open the flusher. Can you see all the way to the bottom -- maybe 6-8" below the floor seal.
There's a lot that can be said once you've made more discovery on this coach. Please get back to us and we'll try to address individual issues.
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