My sweet spot for cruisin is determined usually by rpm's. However actual speed/distance traveled can be greatly altered by the wind and current. Therefore I have always referred to gph, regardless.
We had an issue with a PSV on our water heater. When motoring the engine would heat the water in the water heater to a temperature high enough to cause thevpressure safety valve to pop open and discharge eater in to the bilge.
Cut one big tank up and remove it sections. Replace with small more manageable tanks, then manifold your fuel supply and return accordingly. This is very common in getting the fuel volume you need with minimal mechanical removal.
We recycle from 700 to 1,000 gallons of bad fuel each month. By recycle, we have a company come in and pick it up then they process it.
If you were in Florida we could help. Maybe call the local fire department.