Fuel tank problems

[MR.BOWTIE]

I just recently bought my 71 chevy c10 pickup. The previous owner added two tanks one on each side of the truck.When all tanks are full-I'm a regular fuel tanker waiting to blow.
Well I started noticing that every time I would park my drivers side spare tank started to leak. It would get full of gas.
This told me two things I have a leak in my neck on the tank itself,and probably a problem with the switching unit located by the seat inside the cab on the floor.( a handle I point to what side tank I want to use.
I thought it was because I was parking on the hill. But then it started doing it on level ground.
So heres what I did. I figured since the main tank is higher ,its leaking into my drivers side spare tank,since this tank has a hole it is leaking out all over the place. Due to the hole the tank is getting rust in it,especially since i live by the ocean. Proof by my clogged fuel filter. So I have stopped using the spare on the drivers side,and when I park I put my switch to the spare tank on the passenger side.
BUT CHECK THIS OUT.....Now I'm getting a gas leak from my evaporater up in front of the motor.
My freind didnt believe me that it was gas. He said that the evaporater shouldnt have no gas going to it. Now it looks like it's leaking from the bottom of the evapoater where the gas line goes to the pump.
Has anyone had a similar problem??
Do you think it's cuz of the hole in the one tank??
or is it because of my tank switch in the cab??
How can i stop this from happening. It's making messes where evr I go . If I dont remember to switch it to the passenger side. One time it was dripping from underneath the evaporater and I switch the tank switch from main tank to passenger tank and the drip stopped. Any help appreciated.
SORRY SO LONG,JUST WANTED TO GIVE ALL DETAILS. You guys are awesome

[fine69]

Take the 2 side tanks out. Drain them. Unhook'em. Just use the original tank and see what happens. Then post your findings.