Not to mention that even if they were to choose to use the toilet inside the caravan, the cassette would need emptying at least daily or more frequently, into a designated chemical disposal point. Travellers on "official", designated sites will use the toilet and shower blocks, and also there may be a kitchen/cooking/dining area for that purpose. A lot seem to use their vans chiefly for sleeping in.
As for saying, why don't they use campsites, they are for holiday makers, not travellers, and many sites try to deter them by banning twin axles, German makes, caravans towed by obvious work vans with signage etc. The code is obvious when you know.
Correct me if I'm wrong but I believe councils used to have legal obligation to provide land for traveller sites, often in undesirable brown field areas like the approaches to major railway stations, so by no means always lovely green fields. That obligation was removed some years ago and since then many sites have been repurchased for more lucrative building and the travellers evicted.
I don't know if the council obligation has since changed again.
I don't agree at all with the mess etc. but there's clearly a lot of resentment and suspicion from both sides, and until something changes I don't see that going away anytime soon.
Obviously I'm not addressing all the above to you tvw as you'll be aware of all this, but to others on the thread.