Yep, you won't know what you can carry till you load the coach as for travel, with all the above-mentioned people, fuel, supplies and (worst of all) STUFF.
Yes, the negligible weight of the towbar that falls on the hitch is "stuff."
Load'er up and go to the CAT scale in a truck stop. Tow the Element. The way the scale lays out, meant for tractor-trailer rigs, it'll show weights for Steer, Drive, and Trailer. Front Axle, Rear Axle and Element.
Or, if you haven't bought it, weigh it empty. That'll give you an idea of where you're starting. They're almost invariably heavier than listed. Problem with that is, you'll probably underestimate the weight of what you take aboard.
Good news is that you can take your CAT weights and use a Tire Load/Pressure Chart to get the correct pressure in your tires.
Hint/Teaser: FRONT tire pressure on E450 should never exceed 65PSI.