You could change the fluid in your Granny Tranny every 15 minutes and to no avail. The Key to long granny life is keeping cool and clean. GM wants you to change everything too often just to cover their bases. I recently changed the plugs (first time since new) in my 92 GMC ext cab w/cap ck1500 w/350 . The Mileage was 165,000. I didn't change the plugs because it was running rough, still getting
18mpg----w/o-----15mpg---towing, but I did change them because I just couldn't believe they were not worn out. After removal and examination, they were burning clean with light brown coloration but with a noticeably larger gap on the electrode. The new ones didn't make any difference, but I felt better.

My first experience w/towing airstreams a few years ago was with a 69 23ft Safari about 5500 lbs loaded. I was flying along in the desert with the A/C on happier than a pig in slop approaching Mountain Springs Grade in Calif headed for San Diego, Yeahoo!! Mountain Springs Grade starts below sea level and ends at about 4000 feet, about 15 miles and its all UP. The outside temperature was right at 100 degrees F, when I passed through Phoenix earlier that morning, it was 103 F. Pretty Hot outside, but what the heck, I was cool as a cucumber inside my new GMC, about one quarter way up , people were passing me and waving like crazy, I was happy and Cool so I just smiled and waved back. Suddenly,
I noticed in that funny right side mirror, you know the one that says
"objects appear smaller than they actually are or some such gobblygook", a funny smokey fog that seemed to be following me. It was bright daylight and Roswell was in New Mexico, I was in California, it couldn't possibly be a low-flying UFO or something from a Steven King movie. I slowed down, pulled into one of those turnouts with the water that says "DONT DRINK" for radiators only. What I found was , while the engine temperature was still in the safe range, the granny tranny had gotten hot and expanded the fluid until it was spilling out onto the hot exhaust manifold , making smoke and fog and spitting little droplets out the back, on the Safari and on those people that were wavng like crazy.
How embarrassing. I waited till I was sure all the traffic behind me had long since passed, before I got out there again. Turned off the AC. kept it in Drive2, eased my way up at a much lower speed, tucked in behind an 18 wheeler at about 25 mph and no problem.

Later on that same trip that summer, I went over 8 major passes, Stampede, Donner, several in Idaho, Montana, Wyoming, and Colorado. I just kept it slow and cool. I added a little trans fluid but thats it.
At about 135,000 miles after much towing, I did have to have the trans rebuilt, only $800 here in Texas. The rebuilder showed me the little plastic parts that I had melted that day in Calif which utimately caused the trans to fail 50,000 miles later. Moral of the story as many have already said, get a
trans cooler and keep it clean. Camping world sells a Tekonsha filter and temperature guage combo for about $60, might be worth it.

Enjoy your Tow
Hot Tranny at Large