Check valve in HOT outlet is to stop back flow into WH Tank when WH is bypassed
Check valve in COLD inlet is there to stop back flow of Hot water into cold water supply line due to pressure difference caused by swelling of water as it is heated.....closed loop system.
BUT........neither are REALLY needed
Yes a small amount of hotter water CAN go back into cold water system from WH when the water is heated-------minimal-----and as soon as either (Cold or Hot) water is used the two systems equalize.
My WH has 3 valves.......NO Check valves
Hot outlet, Cold inlet and a Bypass
Hot water MAY backflow into the cold supply BUT this is really a non-issue
RV Mfgs use numerous valving systems when it comes to WH installs
3 Valve, 2 Valve, 1 Valve.......
But when it comes right down to it....Check Valves are NOT needed and are just a cheap/quick alternative to Real Valves.
RV Water Heater Bypass Systems