A couple of years ago, I googled "Liquid Volume in a horizontal Cylindrical Tank" or something similar and got 2 or 3 different formulas. The easiest one for me to reduce to a spread sheet from ASK DR MATH, was:
Consider a circle of radius a (the end of the tank)and imagine that the liquid has reached height h, measured from the lowest point on the circle. Note that 0<=h<=2a. The area A of the segment of the circle covered by the liquid is:
A = pi*a^2/2 - a^2*arcsin(1-h/a) - (a-h)*sqrt(h(2a-h))
The volume of liquid is just A*L, where Lis the length of the tank.
To check the calculation, a verticle tank volume of the same dimensions is much easier, and the volume matches within a gallon out of 240 gallons, which is close enough for automotive work.
I also run a Big Rig once in a while and am concerned about legal weight limits. I have a spread sheet that considers 2 identical tanks on the truck and equates liquid level, gallons and weight. If I can get Richard's tank dimensions and number of tanks, I will email him a printout of what he has.