heh heh... yeah, I thought EVERYONE did this?? Only problem I had was finding the waterbed attachments. Had to find a waterbed store and they're few and far between these days.
I have an '89 MC Prostar 190. I bought an attwood 750 aerator pump that you can get at fleet farm or any other hardware store for about $20. I found a 55 gallon plastic drum. I wired teh pump to the battery and through a switch. I bought about 6 feet of plastic hose and pump the tank full. It takes about 5 minutes to fill and adds around 400-450 lbs. It sits between the motor cover and the backseat so you can still sit in the back if you want. Total cost was probably around $50.