Ken,
There is not a linear conversion for this because the tanks are not symmetrical from bottom to top. It's a simple matter to make your own by draining the tanks and filling in 1- or 2-gallon increments. You won't see anything on the dipstick until about 5 gallons. I made mine to attach to the inside of the fuel caps so they are never lost or forgotten. Also if there is any asymmetry in the way your RV sits on the ground, the dipstick will not be the same for both sides (depends on how picky you are about getting the numbers just right).
greg