More fluid really only slows the rate of temperature rise. So, if you're getting higher temps for bursts something like an uphill slog here and there, then the extra fluid may do the trick.
Yeah I noticed it took longer to heat up in a 30 minute test drive, but I wasn't towing either. It eventually got to 192, but it was also 95 with about 80% humidity that day... so HOT!!! It was about the same conditions last time I towed so I'm not really sure I've remedied the issue yet.
I'm wondering if bypassing the cooler in the radiator might help? Once the engine gets up to temp I'm not sure much cooling is really happening there and if anything it's just helping to keep the trans fluid temps higher, whereas a air-to-air cooler like the HD towing cooler might a better option. If anything bypass the radiator cooler and add a second or larger auxiliary cooler.