I'm always somewhat surprised to see that things like that have such an immediate hit on MPG... with my e250, I've never really been able to tell a difference driving-wise with a roof box/other things on the roof vs. without. But the numbers don't lie.
FYI. If traveling to Mexico and your camper is not titled as such, apply for your vehicle permit online. They are still enforcing the over Gvwr 8600lbs. that you are a work vehicle and not permitted. If you have your permit already you are good to go, and I hear they are more lenient this way. We are titled and registered as a “house car” and took a little convincing to get issued an import permit.