Jack,
 
The basic rule of porta-potti's is simple. The bigger the holding tank, the  
more they hold and the longer time between clean outs. With that in mind, the  
bigger the tank, the harder it is to dump due to the weight.
Be sure to use a quality chemical for the tank. I prefer the brand you can  
find at Wal Mart for about half the price from other marine outlets.
Also, I use a fifty fifty mixture of bleach and water to clean the holding  
tank each and every time I empty it. This will keep it clean and odorless. If  
you get lazy and don't clean it, you and your guests will know the  difference.
 
Rummy