There is a check valve on the hot outlet that checks flow into the water heater so you can winterize. It has a plastic valve inside the metal housing. With age it sticks closed and no hot water will pass. Mine did it this winter in Florida, New valves, with metal innards can be purchased. I just drilled out the plastic parts because I winterize by blowing out the lines and putting anti-freeze in the traps. If you have changed the inlet check valve then this will be similar.
2004 37B Adventurer, 8.1 WH, Ultrapower, DIY CAI, Henderson Track Bar, Tru Center, Koni FSD's, Roadmaster Even Brake, Scan Guage II, 180 Thermostat,