Is that based on the maxi-van? Is so, the rear bumper isn't to close to the rear axle. To much tongue weight with that long rear overhang wouldn't be a good combination. Unfortunately, I have NO idea how much tongue weight would be to much.

Another thing to consider is total vehicle weight. The RV conversion parts have added some weight. The stuff you take with you adds more weight. So it's possible to exceed the GVW of a rig like that. This is a fairly common problems with motor homes.

If weight is an issue, using a tow dolly instead of a trailer would cut the weight a bit.

Edit: Gas mileage would be hard to guess. I've heard of folks with Class B's getting anywhere from 10-16 mpg without towing anything. (half ton vans will get better mpg) How fast one drives will play a role with that much wind resistance. I'd guess it doesn't have an overdrive transmission.

With 3 kids, you might want to consider a Class C instead so you would have more interior room.