Ooooo, you've got the E350 extended van - BIG! I'll bet you have a pretty high towing capacity with that bad boy! We're limited to a wimpy 6600# Luckily our trailer tops out at just under #3k loaded.
We upgraded to the full-size van when expecting #5, and considered the difference between the 150 and 350 and realized it was a much beefier unit. The extra space in the rear (back seat stays out most of the time) is really nice to have. With #8 due in Jan '08, we are certainly happy with our decision to upgrade.
Our first was a '99 gas burner with a 5.4, 3:55 rear end, which did pretty well, but with the Airstream, struggled uphill at times. Our '01 with the 7.3 PSD tows like a freight train. I installed a 4" exhaust, AFE filters, and a mild performance chip and we achieved 22 mpg on our way to Williamsburg, VA last March (no trailer but a fully packed family). (Never did better than 16.5 mpg on highway with gas engine.) Towing capacity with weight distribution hitch is 10K.
Used E-350s depreciate tremendously after 3 years and IMHO are an excellent value. Since '99 (at least), they come with 4 wheel disc brakes.
Below are a couple of pics of the mirrors installed. The second one shows the two additional holes drilled. (The large hole in the middle was the original which was no longer used.) As you can see, I had to add a washer to the top hole. They definitely must be van mirrors, as they are similar to F-250 but not the same.