This is an interesting question. I don't know anything about the Sun-Mar composting toilets, but I have a friend that works for a company producing composting toilets for national parks, wilderness areas and other remote locations throughout the planet.
The key to a good composter is maintaining a high heat to break down the waste, especially the bacteria and viruses. If the location is too high, cloudy or cold to do this, then the composter fails.
With our typical porta potties, chemicals are used to kill the bacteria (and hopefully some of the viruses) and the smell is neutralized until you can empty the potti.
I've had no problem emptying the potti in pit toilets or flush toliets so far. But I would be concerned that a composter wouldn't heat up enough naturally to kill off the bugs in an RV situation. Does the Sun-Mar use an electrical heater?
2006 Baja Tan SMB 4X4 EB50 PH 6LPSD
Mohawk Royalex Solo 14 foot canoe (light white-water)
Mad River Kevlar Explorer 17 foot canoe (flat water)
Dagger Royalex Legend 16 foot canoe (white-water)
Maravia New Wave 13.5 foot raft (fishing and white-water)