I Installed 2 AutoDrain Valves (
Tweetys.com - RV Parts & Accessories - RV Hitches - Warn Products - Search) and I installed the switches on the inside of the bay door bay door. When I open the bay door the switches are accessable without crawling under the slide and kneeling in the mud. The Valves are installed on the push pull rods of the existing valves. So they are installed with the drains closed. It was fairly easy to install.
We spend the winter in our rv so I also put a grey tank switch inside the rv next to the sink in the bedroom. It is always raining when it is time to empty the tank. I also put in a clean dump system with the switch on the door and inside the rv.
The trouble I had was that I could not find any easy access to 12v. So, I put every thing on the same circuit as the water pump. The pump has to be on to use the valves and the clean dump. It has all been installed for over 1 year and works fine.