Let me preface with I'm no expert. I have only made two trips so far, one was towing my friend's Roo 233 from Sedona to Salt Lake City and back, and another towing my X20E from Salt Lake City to my home in Sedona. I have an FJ Cruiser with a 4.0L that I had fitted with a transmission fluid cooler and is rated at 5000 lbs.
One thing I noticed is that on hills, I need to make sure the engine isn't 'lugging'. Know where the power band for your engine is and stay near it. For smaller engines, the higher torque and horsepower are made at higher RPM. It will sound like your engine is working harder than you're used to, but it's far better for it than trying to power on at a low RPM.