Above posts are good starting points.
Water Heater 'Bypass' Valve open or leaking thru.
Connects WH Tank Cold inlet line to WH Tank Hot outlet line.....cold diluting hot out making it luke warm
Showers not turned off via cold/hot knobs but just with button on shower head handle....allows cold/hot mixing inside shower faucet making luke warm water
Your Atwood is gas only model with electronic ignition.
T-stat controls heating cycles.....140*F t-stat opens --closes at 110*F to reheat
ECO (Energy Cut Off---Hi temp t-stat) 180*F opens to shut down heating should Normal T-stat fail. Locks out heating and triggers RED Fault Light. ECO closes at 150*F but heating circuit has to be 'reset' by turning ON/OFF Switch OFF--Wait 30 seconds then back ON
T&P Relief Valve is last safety should t-stat & ECO fail. Opens at 210*F/150 PSI and will BLOW hot water/steam out of valve.
Turn Switch OFF, pull the DC Fuse for water heater, valve out propane....but shut it down should T&P Open