Composting Toilets for Caravans

Caravan World (June 2011) has an article "Dumping the Way", discussing the difficulties of "dump points" for emptying caravan toilet tanks. I was curious as to why Composting Toilets are not in common use for caravans. While they are more complicated to use, the output from them would be much less unpleasant to dispose of.

(Tom, ACT)