On towing in overdrive: I had an older tow vehicle (not for the C-Dory) and, with no convenient way to monitor the transmission temps (and only infrequent towing, so not much motivation to go to heroic measures), I simply obeyed the owner's manual, to not tow in overdrive, 99% of the time (I still would put it in overdrive on long straightaways with slight downhill, etc.).
With my C-Dory towing rigs (both Ford van variations) I am able to run a Scangauge II, which, amongst other things, allows me to easily monitor the transmission fluid temperature. On my previous van, that temperature would creep up slowly if I towed in overdrive (although there was no hunting), so I used that as my guide. If the temp was staying low, I felt comfortable in overdrive; in situations where it would keep on creeping up, I disengaged it.
My current rig will tow in overdrive all day long and the transmission will stay cool as a cucumber. I only disengage it when on a climb where the transmission can't "see ahead" but I can, and I want to keep the RPM's up/sooner than the transmission would figure out it was time to downshift -- or on a downgrade where I am using engine braking (gas engine).
I did have the transmission somewhat "bulletproofed" at a specialty shop, plus I added a larger transmission cooler; but even before that this rig could tow in overdrive without unduly heating things up. It was mostly an elective move.
I can scarcely imagine driving now without the Scangauge II (or equivalent). Much peace of mind in having scads of data at a glance, especially when towing.