I will throw in a curveball. There are tunes available for the V-10 which can allow it to use higher octane fuels and make the most use of it. Supposedly, places like Five Star Tuning have tunes which will allow an engine to use 92 octane, but get 1-2 more MPG from that than the cheap stuff, making it more cost effective. However, that is anecdotal evidence.
I do have to ask: E85 or 85 octane? Totally different animals. 85 octane is one thing, but E85 (15% gasoline, 85% booze) can damage both a generator (Onan expressly forbids using it) and the V-10 (it isn't Flex-Fuel.)