I think that you will all find that the vent hose from the top of the tank is to small to carry enough air for the large high flow truck nozzles. Opening the fill cap on the other side will only help until the fuel level in the tank is above the filler hose inlet which is about 1/2 tank on most coaches. Above that point, the filler hose will no longer vent air from the tank. To increase the vent flow rate, you will have to remove the tank and enlarge the vent hose and anti spill valve.
One serious note of caution though, if you remove the filler cap on both sides when filling the tank, and both filler necks are not at identical height (most coaches are not), and you are pumping fuel into the higher one, the fuel can and will run out the open lower filler neck when the tank is full causing a dangerous situation.
Gerald