2009 Subaru Tribeca

OK, I'm another (relatively) new owner. We are towing our 3023 with a 2009 Subaru Tribeca. We bought it with the towing package and had electric brakes installed. We do have the weight distributing hitch as well. So far we have taken two long trips: one to the Great Smokies and another to Eastern South Dakota (we live in central Indiana). Gas mileage is pretty poor, around 15 or 16 mpg but it seems to pull ok. What I'm wondering after reading the posts here is if we are underpowered. The Tribeca is rated at 3500 and the Trail Manor gives two gross weights, 2750 and 3000 lbs. We do carry a good bit of stuff and are really in no position to buy another vehicle to pull the thing with. Any suggestions?