Sad you have seen that, I live on the Suffolk coast and regularly visit the "hotspots" and also the Norfolk ones too. i have seen plenty of disgusting rubbish left behind by tourists in cars, food waste dirty nappies etc but I can honesty say not by motorhomers. Perhaps you suffer from particularly messy motorhomers but I expect the shortcoming of the few are being used as an excuse for restrictions on motorhomes.
Bournemouth and Sandbanks beaches are a real mess after a sunny weekend but there are not calls to ban visitors because of it. Perhaps it is a different rule for motor homers?