Second question first....30 amp vs. 20 amp. I don't recall ever trying our Coachmen a/c unit on 20 amp shore power but last summer we ran our a/c and limited other items on 20 amp while plugged into shore power in a relatives driveway. It worked fine and we only popped the circuit breaker in his out building once when I guess we tried to run to much stuff.
The valves. In your picture I believe the top two in the picture merely drain the hot and cold lines out the bottom of the motorhome when you drain it for the winter or otherwise want to generally drain down the system or flush it. Not certain from your picture but on ours some combination of values bypassed the hot water heater valve. Since we lived in the North East when we owned ours I typically put antifreeze in our water system during the winter months when it was in storage. I would then used these valves to do various combinations of line flushing, hot water tank flushing and fresh water tank flushing. You may have to "play " with them to watch and see which valves are controlling which lines. As with the electrical schematic I mentioned in an earlier post Coachmen also had a plumbing schematic for our unit. The engineering guys and gals at Coachmen were very helpful when we were looking for copies of these things when we couldn't find them on line.
In terms of flushing the water system I usually connected a hose to the RV in the spring and let water run through the faucets, the toilet etc. making sure to get it to run through the hot water heater and the fresh water holding tank as well. While doing this I'd open various combinations of these values letting the water run out the bottom of the coach until I was comfortable that any antifreeze or old stale water had been flushed out. A word of caution: Before you run your hot water heater make sure there is water in it. You do not want to run it while it's tank is empty for fear of damaging something or overheating and causing other problems or possible injury.
Hope this helps somewhat.
2013 Canyon Star 3940
2011 GMC Terrain SLT-2 toad