While I doubt that moisture contaminated fuel from the plane's tank back to the bottle is actually causing your trouble (I fly in one of the most humid environments in North America, return my unused fuel and never deadstick), don't leave it in the plane's tank. That tank is vented and it will accumulate more moisture and you will have moisture related problems. Instead, consider keeping a second fuel jug on hand to pump it into, one that can be properly sealed. The fuel left in the tank can also cause corrosion of the brass fittings inside (ever notice that green residue that forms inside old tanks, near the clunk?). If I were you, and felt it necessary to segregate the used fuel, I'd carry a small pop bottle or something to keep the unused portion in between flying days. Coke bottles with their standard caps work quite well for storing fuel. Just rince'em out very well with either fuel or rubbing alcohol before first use to get the last vestiges of pop out.

J