Yep what very knowledgeable idroba said. Taking out the
12v pump removes the "block", or check valve that seperates the city water pressure from 12v water pressure. Both sources can run the rest of the plumbing system.
So you gotta cap off the 12v output line preventing water from flowing down the "suction" tube to your fresh water tank.
The drain on my 86 thirty four footer is on the curb side, between the middle and last tire. Caution. It is the world's cheapest plastic ball valve. Turn the handle to drain the tank and your valve will likely break off. At least all the water will drain out of your fresh tank! And since you don't have a 12v pump right now, who cares? Just camp with hook ups.
I would suggest you soak the valve with penetrating oil before trying to open it. Spray it down, go to bed, and try to open it in the morning. This cheap plastic ball valve is pretty easy to replace.
So far my cheap epoxy fix is holding fine. I used Devcon epoxy formulated for plastic so the package said. I rubbed it with 320 grit sandpaper the best I could. I cleaned the fitting very well with alcohol, and it was a dry fitting as I had blown air through the lines.
David