How much fuel you save depends if you drive stop-and-go in the
city or at a relatively constant speed in the country. In
the former case those extra 300lbs will have to be accelerated
each time you go. This costs energy. In the latter case the
300lbs is only accelerated once (or a few times), and the only
extra energy needed would be the minimal increase in rolling
resistance from the 300lbs (assuming it's inside the van of course,
a trailer would add a (little) air and rolling resistance, likewise
would a box on the roof rack).

An upper limit to the increase in fuel consumption would be
300lbs/4000lbs = 7.5%

This assumes all energy is used for stop-and-go accelerations.
Might be close to true if you drive agressively in rush hour.

For country driving my guess is that it would affect fuel exonomy
less than one percent. Maybe much less.

During my time on the east coast I used to tow trailer with a Hobie 18,
a couple canoes and 2 months supplies from NYS to Everglades FL twice
yearly. Those days I still had the original 48hp Diesel. Worked
fine despite the small engine and there was no dramatic increase in
fuel consumption.

Cheers,
Martin