Bill,
I think your biggest problem with the porti-potti is the fact that you went  
with one with the larger holding tank. The smaller ones have to be emptied 
more  frequent, but they afford more head room in that area and aren't as hard to 
 manage when you go to dump it.
Also, the use of a good holding tank chemical will reduce the gas buildup  
that you are experiencing. 
 
Rummy