The load range is chosen based on the weight resting on each tire. Weigh your Mallard, front and rear axles separately and divide the axle weight by 2 to estimate how much weight rests on each tire. The rear axle probably carries more weight. Then consult with a tire dealer who will look up the Load Range needed to carry that much weight. If you can't weigh the rig, use its GVWR rating and divide by 4, then add a few hundred pounds. That should give you are fair estimate. If in doubt or near the boundary between two ranges, go for the higher range.
You need the weight figures to determine how much air to put in the tires too. Every tire has an inflation table which dictates how much air is needed to carry a given weight. You must have that recommended psi to be safe and to prevent premature tire failure. Any more than that psi will cause an unnecessarily harsh ride. The max psi shown on the side of the tire is NOT the recommended amount unless the tire is carrying its max rated load.