After thinking about it, it would be pretty easy to do. Your biggest issue will be installing a sensor or float. I took a look on Amazon and there are several styles of valves that would work. Here is one for $6.00.
The float above is just an ON/OFF switch. For the water, you could use a sprinkler control valves from Home Depot. They are powered by 12 volts when hooked to a timer/controller. There would be no need for a controller, just 12 volts from the coach.
You could mount the float in the top of your tank and epoxy it in like they do with other fittings. Connect one lead of the float to ground and the other lead to one lead of the sprinkler valve. Connect 12 volts to the other lead of the sprinkler valve with a fuse. When the float opens, it will turn on the sprinkler valve,
There are numerous floats on Amazon. You just need to make sure it's wired properly or is reversible. They call it "NO" (normally open) and "NC" (normally closed). These terms just describe what the valve does when it opens or closes. Some will turn power on when they open and some will turn power off when they open. The one I posted should turn power OFF when the float raises.
Lastly, I would make sure that your overflow tube is working properly in case of a failure. If you're worried about a failure, install two of the floats and wire in parallel. They would both have to fail to cause an overflow.
