OK, just an update. Only took a couple of days for a new tank to get to a local dealer. The best on-line price was right at $500, but then added another $125 for shipping. Figured I'd just pay around that and pick it up at the dealer (just in case it turned out to be the wrong one).
The first box they showed me didn't look right so I asked to open it and sure enough it was someone else's order. They did find mine in short order and I was soon back at home prying the old water heater out of it's niche.
Powered down the coach, depressurized and drained the water and pulled the anode to drain the tank. The tank is secured in place by four screws sunk into the framing. There is a trim ring with screws (about 3" long) that tighten the unit and the the ring together. All that is sealed in place but came apart reasonably easily with due care given to not scratching paint.
There were a lot of wires so I wrote the connections down and then, after the water connections were loosened from inside the coach (just enough room to get in there .. a slow process), the unit slid out and on to the ground.
The new unit did not have exactly the same connections as the old one (model ended in a "DE" instead of a "DEL" ...) but after about an hour of snipping, stripping and crimping all the connections were re-made and we slipped the thing back in it's slot. It is a tight fit and I am glad for the help I had to wiggle from the inside whilst my neighbor Danny pushed from the outside.
It notched into place, connected up (with a new check valve ... no more singing! (for a while)) and after carefully wrestling the propane line back in place I seald it all up and it fired up fine.
The electric option switch is cotter-pinned to the 'off' position, so don't forget to pull that out and flip it to 'on'.
Project took about 5 hours all told, including a'my-gosh-it's-cold-and-wet-and-miserable-out-here" break or two.
I think there is a secret microchip on these devices that trigger in conjunction with a sales receipt so that 3 months after the warranty expires they die .. still working on that hypothesis .
Now, about the air conditioner... NEXT!
Thanks for the ideas, feedback and suggestions!