Mixing valve........Used only by Atwood on their XT Series water heaters.
Located on backside of WH TANK directly at hot out with a line coming from cold water to it.
Clog up with crud/scale from hard water.
Remove and soak in CLR or replace (Atwood OEM $160/Honeywell etc thermal valve $90 at hardware stores.
Check valves.....
Cheap plastic plunger/spring/o-ring assembly inside nipple screwed into hot out of WH Tank (hot water line connects to it)
Usually check valves used in-place of shut off valves BUT some RV MFG will install them and a shutoff valve.
If you have 3 shut off valves.check valve not needed.