My neighbor has a Sunset Trail Reserve also, and we went camping this weekend. This was the 2nd time they had used their camper. They said they didn't have hot water so we figured maybe it was electrical. We checked everything and switched over to propane and there was still no hot water.Also we checked to see if any valves were closed or if there was an air pocket. It will be going to the shop tommorow.
The likelyhood of the elec heating element AND the propane burner going out at the same time seem remote. If the tank were split (not winterized properly) someone would have noticed the puddle. The WH bypass valve could be open but you mentioned checking valves. If the WH/fireplace switch were in the fireplace position the elec would not heat...did the other gas appliances work? Could you hear the gas burner attempting to ignite?
We compared the valves to mine and the propan was fine, lit and it was burning. That is what makes this problem he has so odd.
If the burner was lit then the hot water had to go somewhere, did you by chance try the relief valve on the WH to see if there was air or water coming out? Also if the valve was on bypass the water from the tap would be cool as the WH would be "bypassed"...real head scratcher you have there. Good luck