Octane grade for towing

Contrary to layman's notion that "higher octane" equates to "more powerful, the reality is more the opposite of that. High octane gasoline is merely less susceptible to pre-ignition. The higher power part of it is that higher octane allows earlier spark (AKA more timing advance).

If you tested the highest power density fuel (diesel), it would test out well below 80 octane.

As I've said before, probably the best check on power output of your gasoline engine (not diesel, obviously), is to install an engine monitor or perhaps display it if possible (if built in), and look specifically at the spark advance. Higher octane fuels will allow much more spark advance, and thus allow more power output. If your engine is running at 10% power or less, it might not make much, if any, difference.