First, I don't have this setup so I can't be 100% sure... but I think I can make some pretty good educated guesses.
Im the picture I've numbered the valves. (ignore the two verticle lines, they were scribble accidents I didn't catch until I posted)
Valve 1: shuts off the cold water supply to it all.
Valve 2: shuts off cold water into the water heater.
Valve 3: Shuts off hot water leaving the water heater.
Valve 4: Water heater bypass valve.
Valve 5: Water heater drain.
Why all the valves...
Winterization of the water heater.
You don't want RV antifreeze in the water tank so.
1) Turn off water heater (will burn out if you miss this)
2) Drain Tank: Close Valve 1, Open Valve 5. May need to open the over pressure relief valve to let air in (reclose it when done)
3) Bypass tank: Close Valves 2, 3, 5, open valves 1 and 4.
4) you can now pump RV antifreeze into all the plumbing, hot and cold, without it getting into the water heater.
1) purge all the RV antifreeze out of the hot and cold plumbing.
2)Fill water heater: Close valve 4, open valves 2 and 3. (make sure the overpressure relief valve is closed too).
3) Open the hot water side of a faucet, let it run until water comes out without any air spurting/surging. Close the faucet.
4) Turn on hot water heater.
I may have Valves 1 and 5 flip flopped. Look under the RV, which ever has a pipe that just dumps onto the ground, that is the drain valve and the other is the supply. However, the one with blue sharpie all over it, valve 1, I suspect is the water supply.