Atwood water heater problem

I have something strange going on with my water heater. It is an Atwood G6A-8E propane heater. On our last trip it would start like normal and heat the water just fine. After that it would not fire again. I could turn the power off overnight and start the heater later at night to get a shower. Same thing each time, initially runs great then nothing. I have replaced the gas valve, power board, igniter and the thermostat. It is still doing the same thing. Any suggestions welcome.