From what I have gleaned the T1N 2500 tow rating is 5000# all the time. (Some other models are 7500/750.) I haven't seen a mention of deduction using a number of passenger chart, WDH, or other complications. As Boater indicated, you need to subtract the tongue weight from the tow vehicle cargo capacity and monitor Gross Vehicle and axle weights.
A couple selections from the 2006 T1N operator manual. (Around page 223)
"Vehicle and Trailer Weights and Ratings
Gross Vehicle Weight Rating (GVWR) is the maximum
permissible vehicle weight: 8,550 lbs (3,878 kgs),
9,990 lbs (4,531 kgs) or 10,200 lbs (4,627 kgs).
NOTE: The weight of your Sprinter is indicated on the
type plate. For type plate, see Identification Labels.
Gross Vehicle Weight (GVW): comprises weight of
vehicle including fuel, tools, spare wheel, installed
accessories, passengers, cargo and trailer tongue. It
must never exceed the GVWR.
Gross Axle Weight Rating (GAWR) is the maximum
permissible axle weight: "
"Gross Trailer Weight (GTW) is the maximum
permissible trailer weight to be towed:
5,000 lbs. (2,260 kgs)"
"The tongue weight at the hitch ball must be added to
the GVW to prevent exceeding your Sprinter tow
vehicle’s rear GAWR."
That's what I have found. vic
P.S. - As an aside, I just calculated a bit over 19 mpg for a second trip while towing my approx. 2600# (all up) sailboat/trailer. I used the Ciprian Set Sixty CSS method except that I set the 60 using my GPS speed, not by the speedo.