I don't know how practical this is, but your explanation that you're using the tanks for ballast in a boat makes me wonder whether you should be measuring the boat's ride height directly rather than trying to guess the amount of weight in the tanks. This way it would automatically accommodate any variations in fuel load, number of passengers, beer crates etc. I assume this is something you'd need to sort out while the boat is at rest since the ride height will be all over the place while it's moving, and also that you'd need to do a lot of smoothing of the signal because the boat will be moving about on waves and so on, but these problems are all surmountable. If nothing else, this gives you a way to double-check that your ballast is having the effect you expected.