The best way I have found for accurate load/weight is through a truck scale at a truck stop. Take 3 readings
1 - all axles from truck and trailer on the scales
2 - rear axle of the truck and all axles of the trailer on the scales
3 - Trailer axles only on the scales
Addition and subtraction will give you steer axle load, hitch or 5th wheel load and trailer axle only load.
Price will vary but the peace of mind of knowing is worth it. Actually got a reading at a landfill scale once for free.
Have a 2013 320RS. Had weight taken at each wheel on trailer and truck. Camper had about 1/4 of fresh water, all other tanks were empty. Loaded with normal camping gear for a two week trip. Weights as follows: LF=2675 RF=2625 Total front axle=5300 (700 under max). LR=2875 RR=2600 Total rear axle=5475 (525 under max). Pin weight=3100 Total trailer weight= 13,875. Only 140 lbs under max.