I was told when I bought my bus to fill the water tank till it overflowed and done this ever since. Now en-route i alway loose water and generally arrive with about 3/4 of a tank on the gauge. Am I overfilling to start with? and if so why do i not just loose the overfill rather that the tank gauge telling me it's only 75% full then. If I fill it to overflow on the pitch it doesn't loose any water. Ideas???