Identify and correct tire pressure

You rig should have tire pressure recommendation plate - look around. Use that tire pressure until you can get the rig weighed. When you weigh make sure the rig has typical weight when traveling (fuel, water, and something that emulates the weight of the typical number of passengers). I used a truck stop to weight my rig - they had nominal charge and the weight machine only determined weight of front/rear axles which was acceptable to me. Michelin website has nice table which converts weight to tire pressure based on size of your tire.