Goodness! My 28' RV weighs less, and it's got a 460 under the hood!
Anyway, you're bound to heat up when pulling a healthy load. I'm not sure 1200 is much to worry about unless it is there for a pretty long time, which may cause engine temps to creep up. Just watch to be sure you're coolant and oil remain within their limits. That's all I ever paid attention to when hauling a 9' tall trailer. If it got over 230ish, I'd consider stopping for a brief cool down. 250miles at a time was no trouble. Oh, and transmission temp... Make sure that doesn't go too high. It's even more of a concern than the engine.