when you take your donor trailer apart, you might find it easier to pull the flooring off to get the tanks out. Spend some time looking over the setup and it'll explain the dump valve(s). And talk to local RV/trailer shop folks, they can usually help with advice, and they'd be local since I can't make the trip
Many RV tanks were installed while building the trailer, so they can be surrounded by walls and flooring n' stuff. Get ALL the stuff out of the way before trying to pull the tanks, or they will break when forced. This means taking the trailer apart, something you'd probably do anyway.
SOME tanks were bolted in from underneath using hanger straps, so might be removable that way.
Try to flush the tanks before pulling....get as empty as possible. Having the trailer level helps.
1) First FILL the tanks. FLUSH the BLACK tank BEFORE the grey tank.
2) THEN flush the grey tank....the idea is that the grey water helps flush the last of the black crap out of the drain hose. If there's a black tank shutoff, it might behoove ya to close it before flushing the grey tank...your mileage may vary.
About your starting problems: Make sure that when you turn the key, you have voltage going to the solenoid. IF you have voltage going into the solenoid, then test the voltage coming OUT of the solenoid. Solenoids and electrical connectors can get rusted, and pitted contacts internally, which can be the problem.
Then if the solenoid is good, pull the starter and have it checked out. You can usually test it on a bench with a battery (make sure you can pull the wire away!), or take it to NAPA or similar to test it (safer to have them do it if you don't know what you're doing). Disconnect ground wire from battery FIRST before pulling the starter!
Batteries should be around 13.2 volts when fully charged IIRC (each cell is actually 2.2 volts X six cells=13.2 volts)...I could be wrong, there's plenty of battery websites out there to double-check. Low voltage could be a problem with a weak connection (corroded) or pitted contacts in a solenoid.
And I'm sure by now someone told you your old water tank is for a completely obsolete pressurized system. They stopped doing those when decent water pumps became available, and check valves prevented city water from back-flowing into the newer plastic tanks.
Hope this helps.