Take the wheel position axle average and add 15%.
2,150-lbs per tire GVWR, would be about 2400-2500 lbs.
The only accurate way is to weigh the TT, wheel-by-wheel, loaded for travel, full up. As any one of four on a tandem axle trailer is likely to carry more than the others the heaviest wheel position is the one to work from.
But you won't know what it is, or how far off it is from the others without 10 minutes of work.
Stick with commercial LT tires, etc, etc
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411