The problem with all of the what can I tow programs I have seen is they assume a pin weight of 20 to 25% of the total 5er. That is often not true with modern units, some are under 15%.
The only way to know for sure is to weigh after loaded ready to camp. But you can guess closer by doing the math on dry weights, and use that percentage, perhaps increased some as much of the added/wet weight tends to be in front of the axels, as that is where most of the storage is.
It will get you a lot closer than using 25% with an actual dry weight factor of 17%. You can always distribute your load, travel with empty tanks, and more importantly don't load it up with junk.
