This is a great forum! I have owned a 1996 190P and 2007 210P Roadtreks and I never knew that somewhere between those years that RT had a gas/110v water heater. I looked up the 2004 brochure specs on Roadtrek.com and sure 'nuff.....there it was: a two way water heater. I looked at the surrounding years and the dual lp/110 is shown for 2003, 2004 and 2005.
I have had to replace most of the GFIs in our 2007 RT over the years. They just seem to fail. Recently, I had an electrician tell me they were having a dickens of a time with GFIs because they are virtually 100% made in you know where now and have high failure rates. He said he had three in a row out of the boxes fail. Wow.
I think I would cut the power, replace the GFI and then see what happened. I am surprised that the water heater won't apparently work on propane even if the 110 circuit/element/whatever is failing.
Roadtrek was always trying to take a few models for the first several decades of its corporate life and just tweak and improve them every year. We will keep our 2007 210P until we sundown because we love the quality, twin beds, Chevy chassis and really great cabinetry. Same applies to your 2004 190P.