It is very possible that you simply have the vent line on the fuel tank plugged. I've actually seen them go to the point that they will start to try and suck the tank in.

In the engine compartment, on the floor right behind the engine, you will see a plate that has a couple bolts in it. Take that off and remove the piece of foam, if the varmints haven't already done it for you. There will be a small rather flexible rubber hose that is the vent line. It probably has a chewed spot in the side of it, and that will be plugged with crap. The other end of that line is located up the side of the radiator behind the hinged panel, and will have either a single of two breather caps on it. "Usually", there is enough hose there, that you can re-route it, cut out the bad spot, re-attach, and your back in business.

This becomes a yearly maintenance spot, in pull up the cover, clean out that area, take the foam out, and usually leave it open. I've seen this happen to more than one machine, and once it starts to pull a vacuum on that tank, you will get the results you are describing.