With all that stuff why even care what the tare once was and never will be again..... I cannot understand this thing about Tare, it is meaningless as it doesn't exist, the main thing is ATM if your over then you must throw stuff out and what you need to do to weigh your van properly is get the tow ball weight, then subtract that from the ATM that will give you the maximum GTM you can have at that point in time, you will notice that the GTM on the plate won't match as the ATM is a combination of tow ball and Axle weight, as the tow ball weight increases the GTM decreases, the weight has to move and the ATM cannot change..... Same with most tugs as the vehicle weight increases the tow ball capacity will decrease, depending on vehicle.
Of course every time the tow ball changes so does your GTM... its easy once you get the hang of it all.......
Your 220 kg ball weight will rise a good 30kg with the genny, the Catch 22 is you add 40 lts of fuel at the back to counter balance, again distance etc matters, and you find you have blown the revised GTM........ also dual axles don't throw the weight like a single......
VAN: ATM minus tow ball weight equals GTM ....................... TUG: GVM minus tow ball weight shows how much bum room you have left.
PS: If you buy those scales you can come up here and I'll take you for a boat ride,

have a nice set up for a van to park here.....power, water, dump point.... doubly welcome if you can catch fish .................................