5.4L, 6.8L or 6.0L Diesel? 3.55, 3.73 or 4.1 gears? E-350 Super Duty or E-350 Super Duty Extended?
How heavy is your trailer? What is the hitch weight?
Depending on the van's configuration the simple towing capacity can be as low as 7,200 pounds or as high as 10,000 pounds.
If you have the diesel, maintaining speed is not a problem, hills or wind. The V-10 can be pretty impressive with loads under 10,000 lbs. I've had both but with weights above 13.000 lbs I am with the diesel.
Of course getting the trailer weight matched to the van is more than just looking at the towing capacity rating. You have to check the GCWR, and rear axle rating as well.
Here is a link to the 2008 van specs: E-350 SuperDuty Towing