I had a similar problem with my 1988 Prowler. I bought it from a neighbor and it was not winterized and all the plumbing froze.
I ended up replacing my whole 6 gallon Atwood water heater, tank was cracked in 3 or 4 places and the insulation had been removed at some point and was missing.
Once I got the new water heater in, and tried to pressurize the system, I found the inlet valve on the toilet was broken and spraying water everywhere.
Also every drain trap, including the one under the shower pan was broken.
Luckily I didn't have to replace any pipes.