They must have.
There are many sites and motorhome servicing points in various places (some of which require a small payment) which make it easier for Moho owners to do the right thing - schools, ferry terminals, visitor centres etc.
Yes, there are plenty quiet places for wild camping too. I think many of the Rental guys tend to head to a few hotspots. Last September there was a convoy coming off the ferry heading for the Butt of Lewis which was rammed with vans.
There are sites on Barra and Mull and dotted about the Uists with more appearing. Add the bunkhouses, pods, and hostels the Islands are becoming well equiped for Mohos, cyclists and walkers alike.