GM-GMC: CHEVY VAN TOWING, OVERHEATING, temperature guage, clutch fan

Question
I tow a 5000 lb travel trailer with a 2005 Express 3/4 ton van with 6.0 engine, 3.73 rear, 4L80E trans. When pulling over a mountain pass the temperature guage goes up over 240 degrees, but clutch fan never seems to engage, temp drops immediately when going back downhill. Have had the clutch replaced under warranty but didn't make any difference.  Service managers at dealer seem to think this is normal, which seems ridiculous. I do have an aftermarket tranny cooler, so hopefully am not hurting transmission.

Answer
Excessive heat is not good for anything.  At first glance the tranny is adequate as is the engine for the trailer weight, idealy you could have a 4.11 rear but wouldn't make that much difference is the hills, more for getting rolling.  No doubt your engine will be working to pull a trailer of that weight through the mountains, I am hesitant to suggest any modifications to your van while it is still under warranty.  That being said, electric fans or a larger radiator may help if you can find them to fit your application, neither will be cheap.  My only other recommendation would be to turn the heater in the van on hi and on full heat, it can help reduce the running temperature, perhaps only by a small amount but it will help some.  I will do some inquiring and if I find anything else out I will update the post.