Only the second-row seats come out of my Sienna (the third row folds into the floor), and I do generally leave those seats out. As Jack mentioned, without a weight-distributing hitch the trailer adds load to the van; the rear axle
gets all of that load, plus more pried off of the front axle
by the lever effect of the hitch being behind the rear axle. I am generally more concerned about excessive load on the rear axle than inadequate load.
With my Sienna, I measured the almost-empty axle weights (one occupant, no second-row seats) at 1130 kg (2486 lb) front and 870 kg (1914 lb) rear. My Boler
is a 17-foot B1700RGH (about twice the weight of a typical 13-foot egg) and even dry and nearly empty it had 100 kg (220 lb) of hitch weight.
The distance from the rear axle to the hitch ball in my case is about 40% of the wheelbase, so the trailer pushing down of the hitch like one end of a see-saw (with the rear axle as the pivot point) lifts 40 kg off of the front axle. Hooking up my B1700 to the Sienna added the 100 kg of hitch weight plus 40 kg of transferred load (140 kg or 308 lb total) to the rear axle.
- front axle goes from 1130 kg (2486 lb) without trailer to 1090 kg (2398 lb) with
- rear axle goes from 870 kg (1914 lb) without trailer to 1010 (2222 lb) with
The van balance is fine with or without the trailer, and more cargo in the rear is not needed. When the cargo we want to carry is considered, we are certainly better off without the seats. I have even considered removing the third row set for heavily loaded trips.