Most motorhomes have quite a long filler hose from the nozzle opening to the tank, and this hose doesn't have very much drop. As JVJ says, there may be a kink in the fill hose, or more likely a sag. The fuel collects in the sag and backs the vapors up to the nozzle and causes it to shut off.
If you can climb under the back of the motorhome, try to follow the filler hose and see if there is a sag. You may be able to straighten out the sag and support the hose by attaching it to something under there with some zip ties.
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