Frankly I am surprised you can tow that much weight with the 3.9 and the A-500 without the transmission impersonating the bottom of the Fukushima reactors. I've never really towed anything with my full size van with the 5.2 and the A-500, but I did have to have the Tx rebuilt at 130K miles, shortly after my acquisition of it. It was apparently used to tow a boat from Florida to NY once a year. It had a trailer brake controller, and had no additional cooler installed. No idea if the previous owner kept it out of overdrive when towing, but it runs over 3000 rpm at 65 without OD and 2200 at 65 with OD and lockup.
That TX rebuild cost nearly as much as the Van did, and I've been trying to insure that it does not fail prematurely. I wish I had a stacked plate cooler instead of a fin and tube cooler, and I wish I had actual data on temperatures it achieves. If it regularly flirted with 220f plus I would likely first try the stacked plate cooler, and if that did not significantly lower temps I would add a powerful fan to it. There are all sorts of TX life vs ATF temperature data all over the web. In your usage I would be ensuring maximum cooling potential.
Plus one on the 'excessive' fluid refreshing schedule. They do sell deeper pans with finned heatsinks and a drain plug. I was not willing to lose any ground clearance for a deeper pan.
I recently replaced my steel lines and added the derale external filter mount, and it wound up requiring close to 7 quarts of new ATF. There was a noticeable difference in shifting, being faster and firmer, but especially the 2 3 upshift. The old fluid was still red, not quite pink, and smelled sweet, not burnt at all.
I am awaiting a deal on ATF+4 and will drain 4 quarts through the plug and replace with fresh as I want to make this vehicle last as I do not care about status or latest and greatest or the opinions of those who looks at a near 30 year old van and think what they are programmed to think.