I sure don't know everything, but every water heater i have seen has a drain plug.
the low water valves just drain the water that is in the lines.
you will need to open the pressure valve to allow air into tank, so the water will drain out.
do you have the manual
Probably won't drain all the water out. A little water in the bottom of the water heater won't hurt anything. The little water in the bottom will expand as it freezes and not cause a problem. Think of a soda bottle with a little water in it freezing. No chance the bottle (or water heater in this case) will rupture
If you open the true low-point system valves, and open a hot water valve inside the rig, yes, the water heater should drain. It'll be slow, but it will happen.
Quicker and more definite to remove the drain plug. Not familiar with Rheem WH in an RV (I'm only familiar with Atwood or Suburban). Is that a tankless system?
I didn't think it could happen, but someone on here mentioned it once, so I (and a few others) actually tested the theory. We discovered that a tank WH will in fact drain using the low point drains on the water system, but air should be allowed into the system from a faucet. So if you have a tank WH, and you open the true low point drains, and crack a hot water valve somewhere else in the line, the WH should empty through the LP drains.
But you're absolutely right about checking the manual. OP should find the manual for that water heater and check it out.