I'll agree with many of the comments already posted. As long as it's a "conventional" trailer, don't expect much difference in mileage regarless of combo.
the mileage difference between towing my 7x14' cargo trailer and 35' outback is 1 to 1.5mpg and the big difference is the much smaller frontal area of the cargo trailer.
yes, the hybrids like the trail manor will give increased mileage, several friends that have them verify that and reall like them. One downside is that they are NOT easy to use at a rest stop for a quick lunch, and do take more time to set up when you hit the campsite. Setting one up or taking it down in the rain is not the most pleasant thing to do. Course the big trailers have downsides as well.
We travel close to 8,000 miles/year, camp close to 3 months/year. What we have found and verified with others is that even at 10mpg or so, fuel accounts for about 25% of the variable costs of camping. Campgrounds, entrance fees, groceries and all the other stuff you do add up. So we really don't sweat the fuel costs.
IMHO Especially if your going to travel much, get a tow vehicle and trailer combo you like. Tow vehicle that is comfortable and takes you places you want to go when camped, and a trailer that is comfortable for your camping style.
IMHO you'll be much happier than optimizing around MPG and sacrificing on the tow vehicle or trailer.
