In some cars, that is sometimes a sign of a bad evaporator canister. This is a canister full of charcoal which is intended to catch the fumes that are not ingested by the engine so that they are not vented into the atmosphere. When the canister is full of crud, or one of the check valves goes bad, the fumes no longer go into the canister, but back up the filler tube. It is this back up which causes the slow input of gas into the filler tube. I am not saying this the problem with your motorhome, but if you have such a canister, it is something to consider.