I believe the issue with sign-written vans is not to do with the travelling community, who as posted use plain vans, but to do with site licences which often are leisure only and don't permit tradesmen working away from home - but plenty of sites ignore that.
Personally, I see no reason why tradesmen with sign-written vans should be precluded from a weekend, or longer away from work, as some of them won't have another vehicles capable of towing.
I've never come across the travelling community on a caravan site in over 4 decades of caravanning - but our local council spends a fortune clearing up after they descend on some of our local football pitches and parks!