I have generally been quite happy with the porta potti supplied by Parkers, although as anticipated the electric motor contacts needed a bit of maintenance after 6 years. However the top tank (fortunately not the bottom) has developed a slow seeping leak. I wasn't happy that I could seal this with a crack finder, so we decided to replace.
This note is a warning, Porta potti seem to be discontinuing this model in favour of a 'cube' which doesn't fit the molding provided by Parkers, though you can still get a replacement. I've actually bought a 365 model (manual pump not electric) from Force 4 but if you have one which you intent to replace in a year or two, might be worth doing it now!
http://freespace.virgin....james.hamilton/forum.jpg