You should ask on one of the Casita
Forums. There is a check valve on the newer Casitas that needs to be tapped to open up. My 1997 didn't have it. Good Luck!
I tried a search and this is what I came up with. Somewhere there is a photo, but I can't find it. It is a check valve that gets stuck. Here is a fix:
"If you say that the propane
burner cycled on, heated the water and then no water came out of the hot water tap then you have a stuck valve(s) at the rear of the hot water heater located one above and below the bypass valve. You need to pressurize the system and tap the valves with the handle of a screwdriver or other object to jar them loose, common problem with these valves."