Pulling a travel trailer

[crawdad]

I've got a travel trailer that weighs around 5500-6000 lbs. The other day I pulled it to a hunting lease in another state about 275 miles away. I had heard that I shouldn't pull in overdrive so I was pulling it in Drive. The road was hilly, but not all that bad. I noticed the truck getting pretty hot, especially on the hills. Seems like it was up around 210, sometimes higher, but it never did get in the Red. The engine RPM's were up there pretty high in Drive and I figured that might have something to do with it running so warm so I dropped it into Overdrive and the temp. went back down to just above normal. It normally runs right around 180 degrees. This truck was not originally set up for towing, but since I only pull the trailer about two or three times a year, I didn't think it would hurt it. My question is what can I do to keep the temp in the normal range and what can I do to the truck to make it a little more capable of pulling a load. I've already put a tranny cooler on it. I'm not sure what gears I have in the rearend, but I would suspect they're pretty high. Any comments appreciated.