First, pump the dirty fuel out of the tank. If you can rig it, run the fuel through a filter system and you can probably reuse it? If you have two tanks
pump all the fuel into one tank, clean the empty, then through a filter back to the empty then clean the second tank.
If you can access it, steam clean the empty tank. If you can find someone with a portable rig on a truck, maybe a carpet cleaner for the hotels and a long hose you can poke the wand through the little hole in your tank and get at least some of the crud out. Thoroughly rinse out the tanks
in case there's any residue from the steamer, dry and refill using clean, filtered fuel and your favorite fuel additive/biocide.
If you aren't happy with the old fuel, even after you filter it, I'm sure you can find a local that will take it off your hands.