I can add to my previous comment:
I've towed my 5500 over 1300 so far, with my '06 Highlander V6 AWD (3.3 liter engine, 5 spd transmission).
Our trip brought us over some the hilliest terrain in eastern Canada: the Charlevoix region and the Laurentides wildlife reserve.
I've weighted my trailer at the start of our trip: 3235 lbs loaded. We had full propane
, food, beer, etc. so it won't ever weight
much more than this.
Towing was always done with OD switched off. Most hills required a downshift to 3rd, keeping the rpm between 2600 and 3800 (peak torque for this engine is 3400), cresting at 70 km/h or more.
Just like everybody I could use 50 more hp, but I never felt underpowered.
Fuel mileage, recorded with my calibrated Scanguage, averages about 17l/100km, which translates to 16.6 mpg (UK) or 13.8 mpg (US). Not bad considering the hilly terrain, and within my expected range.
Rig drives nicely, very stable (I'm using an Equal-i-zer WDH), and I'm under all my weight limits.