Also, re not using the toilet in the trailer, is more to do with the proximity of the toilet to where you eat, and the fact that chemical toilets need emptying (obviously) and can leak. This is why on sites everyone usually has their own toilet outbuilding.
Very few Travellers leave their waste, human or otherwise behind for others to clean up...it's just those are the experiences of Travellers that most people talk about. Lots of non Travellers throw their rubbish in the street and fly tip etc, look at the mess left behind after music festivals, for example. Also, when Travellers move on, some local people see it as carte blanche to fly tip in the area the Travellers have just vacated (white van men are notorious for this!!) and the Travellers get the blame.