Mixing Valve used on Atwood XT Model water heaters
10 gal tank.heat water to 160*F then hot out goes directly into Mixing Valve where cold water mixes and outlet temp is controlled to 130*F
Equivalent 16 gal of HOT water from a 10 gal tank......
Mixing valves clog up due to hard water minerals and then NO HOT WATER
Some folks have had success by removing Mixing Valve and cleaning by soaking in CLR, HOT Vinegar etc
Personally I do not like them due ---PITA
Atwood OEM Mixing Valves are expensive ($140)
Atwood 90029 RV Water Heater Mixing Valve - GetRVParts
Atwood with Mixing Valve
And YES many RV MFGs install check valves on water heaters instead of shutoff valves
Common on HOT Out ---Winnebago likes one on COLD IN & HOT OUT
OEM water heaters not shipped with check valves-----RV MFGs add them