2500HD 6L gas 2004 not shifting into 4th when towing

[Hoosier Deer]

I have the truck listed above. I have a 7000lb bumper hitch camper (empty). With firewood, camping gear, etc, I'm pulling about 8500-9000lb. I find that my truck, when in tow haul mode, will stay in 3rd for very long periods. Rarely does it shift into fourth (drive/overdrive). Does not matter the terrain. Flat, hilly, doesn't matter. This keeps me at 2400-2500 at 53 mph. If I push the gas and rev it up to higher rpm nothing changes. After awhile my engine temp will rise because of the higher rpms. I have tranny services performed every couple of years. Truck shifts to OD normally when not towing. We camp 5-6 times a year and never more than 2hr drive. I usually just back off and drive slow but it's frustrating knowing I have more than enough truck for the camper I'm pulling. Anyone else have this problem? Suggestions?

[2017HD]

While tow/haul does not lock out 4th, it does limit how the transmission shifts. My only towing experience with the 4speed was a 99 Tahoe and it also stayed mostly in 3rd when towing my 6000 lb TT. That old 5.7 did not really like to rev like the 6.0L. What happens if you take it out of tow/haul?

[Hoosier Deer]

Tried that today and it acted similar to when the tow haul is on. Up until the tranny temp gets to 150-170 it will shift to fourth. Once it gets above 170 it quits doing it. Tranny Temp while pulling usually works up to 190-200 and stays there

[elchilero53]

You said that you service the transmission every two years? Shouldn't you have it done more frequently? Especially towing and hauling payloads at or near your payload capacity - 7000 lb trailer + 1500 to 2000 lb payload in the bed + 700 to 800 lbs tongue weight + plus passengers/pets (= 2200 to 2800 lbs payload + 7000# trailer). You are probably below the max tow rating but have you exceeded your payload capacity rating? That might cause the truck's transmission to shift oddly and get hot.