Over the years I have cleaned many plastic and metal fuel tanks with Marine Motor De-Carb from ValvTect and similar products. It is primarily intended to be sprayed into the carb of a running engine for the purpose of removing carbon buildup, however it is very effective for cleaning small fuel tanks and carbs. It is in an aerosol can and can be sprayed into the fuel tank and let it set for a while and then can be washed out with water. I have a Case garden tractor coming next week for service and the owner says that the fuel tank looks like a can of dried up varnish. I will have to do repeated applications on this tractors fuel tank to clean it up and it is a 3 gallon plastic tank. I'll have to do a lot of other stuff to this tractor before it will be running, but the fuel tank is a priority after I check out the engine which is a Kohler K 321 14 HP single cylinder.
There are other companies that package the same product, Johnson/Evinrude, Yamaha, Mercury Marine package it and some car companies do it also. I used to buy it at a local Ford dealer before they closed up a few years ago.
Mad Mackie in CT