two things you can do
crawl under the coach and look for tubing. The low point drains will have two tubes, a red for the hot line and blue for the cold..... well actually the colors may not be there for all coaches, i don't know.....
anyway, these will be open ended tubes sticking straight down.
Once you find that, you can chase down the valves just above inside.
In mine, I have a set under the bed and another set under the kitchen.
for the bed, I have to reach down through a wad of wiring to see two plastic valves.... pull the handles up/out not turn.
In the kitchen, I have to remove a drawer
The other thing you can do is to call Thor's customer service and request the plumbing drawings that will likely show them.
for the water heater, what I do is turn on the outside shower, which in my case is very near and just below the heater, then pop open the TPRV to vent the heater. I know the standard thing is to pull the plug, but the outlet pipe sits at the same level in the tank as this so i figure it's pretty close to the same thing.....