Just a guess, but if you only use one bag it would be harder to control. IE all the air would go to the side with the lower wheel weight. It might want to rock back and forth between the front and rear axle. Fill a zip-lock bag with water and play with it. You'll get the idea. I believe you will find you need separate control/bag for each wheel.
Just to be clear here. Are you making air bags to go under trailer tires to level the camper or making your own air bags for suspension on your truck? If talking about air bags on your truck I have no comment.
2012 Sundance 3100RB w/Reese Goose Box
2004 Ford F350 6.0 L PSD, CC, DRW, long bed, B&W drop ball hitch, Firestone Ride-rite air bags.