burlmart - As Cowboy66 mentioned, we got our fisheye at a RV dealer also.
A common drain really would not work too well as your water heater is an actual pressure tank with cold water going in and hot water going out with some kind of heating element. Nice idea, but would require some engineering.
We have never had a problem with our refrigerator when the RV is on a slight incline. Really would not be comfortable sleeping if it was on a significant incline, so one problem is solved by another.
You are correct on the pressure regulator. You would eventually build up the coach pressure to the exact same value as the supply line pressure. How important is the regulator, well it is a small price to pay compared to a blown line that is hard to repair.