You could do 6 in a Class A if you have enough slides, I have a King (I had a queen in Master in my '99), couch has air mattress which is a queen, and other couch makes into a comfortable full, but the back/seat split wouldn't make it real comfortable for adults in my opinion. I carry a twin and queen Intex Supreme airflow mattress with built in compressor
Intex® Supreme Air-Flow Bed : Cabela's
which fits on the floor and there is still sufficient room to get to the bathroom.
I agree if you are going to be sleeping more than 4 regularly, you may want to look hard at a Class C or Super C, may fit your needs better than a Class A. Good Luck!