A trailer that size, with only a single axle, for braking, will heat the brakes up pretty quick, on a steep downhill. The tow vehicle engine braking, by putting the transmission in a lower gear, going down, will help to keep the brakes cooler on TV, and trailer. There is no easy fix, and a V6 engine, will not slow the rig like a V8. If you back off on the trailer brake setting, you risk over heating the TV brakes, which could cause them to fade, and a more dangerous situation. Going down slowly, similar to the speed going up, is your best option.
Wind drag is always going to hurt mileage. We all deal with that, and towing at a lower speed is about the only fix for that too. Most gas engine TVs will get about 8-10 mpg, at around 60 mph.
It seems you know, when the time comes, you should get a heavier duty tow vehicle. When you do, you most likely will want a bigger trailer! :)
Jerry