When we did our last PDI (took about 3 hours) the dealer rep showed us where the hot water heater was, how to change the thermocouple (now we always carry a spare), and how to drain it for the winter. I would have thought that when they showed you how to service the water heater, they would have noticed that there was no way to access it.
As for Jayco, just what did their CSR say when you called her. She has always given me a incident number to refer to when we have follow-ups about the issue.
I say "she" 'cause it's always been a Lady who answered the CSR phone at Jayco. They always want your VIN or Serial number, so have it ready...
If they didn't respond to your phone contact, the next step would have been a registered letter. I had to do that to get Starcraft's attention.
Sorry that you had issues.