On the self build camper thing it could be that caravan parks or their insurance provider consider that they could be a fire or explosion risk. How many self builders get certification for gas and 240v electrics? It may be down to the small print in their public liability insurance.
The question is are those who have “acceptable” motorhomes and caravans prepared to pay increased site fees to cover increased caravan park insurance costs to cover those vehicles currently considered a potential hazard to others on the site?
It may actually be totally impossible for the various camping clubs to obtain public liability insurance which includes cover in the case of an incident involving self build vehicles.
I have no problem parking alongside self builds when wild camping as clearly the owner of the vehicle alongside would prefer it not to explode and the owner hopefully has some form of insurance cover in the event of the worst happening. But when it comes to public liability insurance which campsites have to have it it a completely different ball game.