There is a process called "fuel polishing"--mostly applied to diesel. Basically the fuel in the tanks is recirculated thru filters with a pump.
Yes, Startron--(Soltron) has been used. It does definately seem to work in diesel fuel. I have also tried it in gas, and it seems to work there--but I don't have the dramatic proof I had in diesel. A friend's boat had gelled diesel (he forgot about one intragral tank and the fuel had been in the tank for 30 years--the tank was "found" when he repowered recently. They could not pump the fuel out--so Soltron (Startron) was added to the tank--the boat sailed across the Atlantic and the diesel was then liquid enough to be easily pumped out--the tank was clean, after that fuel was removed and fresh diesel (with Startron added) pumped thru it. The boat had been almost continously sailing all of these 30 years--and covered about 150,000 miles during this time, including several circumnavigations)
I had some "missing" with my 18 footer, which had unknown history with the tank--and the Soltron resolved the problem. (Plugs and injectors were OK). So, I do use it.
One of the problems we will be seeing is with the addition of ethanol, there will be gunk loosened and cause plugging of the filters. I would try the Soltron, and pumping out the tanks. If the fuel is real old--get rid of it. I used to physically clean out my diesel tanks (man holes)--I have never done this with gas tanks--mainly because I don't have man holes in gas tanks...