1) ensure your clutch fan is working properly
2) If you have an external only transmission cooler, look at converting the transmission cooling to a two stage system (Radiator with a built in transmission cooler, that pre-cools the transmission fluid, then is plumbed through an external transmission cooler before returning to the transmission.)
3) Ensure that the the transmission lines are 100% clear of the debris from the two previous blown transmissions.
Another thing... don't worry so much about maintaining speed up hill... towing 9,000 lbs... worry about working in the capability of your vehicle... keep the RPM's in the powerband and let the Van work.. If it loses some speed...so what. Its okay...
fianlly, Your TV is 22 Model Years old now... Don't expect it to perform like the new trucks... they have 360-400 hp and 6-800 lb ft of tq... Massive cooling systems.. and 6 speed transmissions. You have about 245 hp and 400 ish lb ft of tq... similar specs to a new Ecodiesel Half ton.
Thats about all I can think of right now.
Thanks and good luck!
Jeremiah