Ten bucks later, the Beast now has two working door locks. The deadbolt worked all along, much better after a squirt of teflon. The previous owner reported that he'd never been able to get the lock on the door latch to work. Turns out that lock was on a different key! It took a while for the locksmith to figure out the right key. It took 5 minutes to make two of them.
Fresh water tank(s): I drained both tanks: the "master" tank and the hot water tank. It took more than an hour for the big tank to drain. I measured it as best I could, and a little conversion math ( one cu ft = 7.48 gallons) tells me that it holds 40-45 gallons: more than I thought! The hot water tank only took about 20 minutes to drain, but that was more than I expected. There's nothing to measure on that, so there's no real way to infer its size.
Maybe tomorrow, with the water tanks empty and the gas tank almost full I'll take the rig to a truck stop and weigh it. And then refill the tanks with a sanitizing solution and drive it around to slosh for a while. Drain, rinse, repeat.