Mandy,
If I understand your question correctly, changing the inlet size won't fix the problem IMHO. The major issue is the size of the fill pipe and inlet to the tank. The cross sectional area is not large enough to accommodate the volume of fuel that is introduced by a pump with the handle nozzle on full. Going to a truck nozzle will make the problem worse as it will try to force even more fuel down the pipe. Another issue may also be at play. On many of the older coaches, the fill port into the tank was located 25% to 40% below the top of the tank. This was necessary to achieve a gravity assisted flow because the tank was mounted relatively high in the chassis in relation to the fill door. Thus, at some point fuel is not emptying into the tank above the existing level of the fuel in the tank, but is being added below the level of the fuel in the tank, restricting the amount of fuel one can push down the fill pipe w/o significant back pressure/ fuel backup.
Gerald may have more accurate info on this.
Steve