This does not make sense and only adds confusion for the OP. The passengers, cargo and snacks are not going to take away from the maximum trailer weight -- they will take away from the payload of the van. The only place the passengers, cargo and snacks in the van are related to the trailer weight is when they are included in the GCWR.
Rubber Ducky has it right. Tow ratings are based on an empty vehicle. Any weight added to the tow vehicle, other than the 150lbs allowance for the driver already included, reduces the tow rating pound for pound.
