Bottom line when towing a 5th wheel there will be very little weight added to the front suspension. Example my hitch is set full forward and I only add about maybe 150# to the front axle. So do the math weigh a 2500's rear axle add 600# to that number for toolbox and hitch, now subtract that number from what the max weight the two rear tires can carry at max inflation. Take 25% of the RV's GVWR add that number to what the theoretical weight of the rear axle should weigh. Are you more or less than what the tires can carry?