There are a couple of reasons why manufacturers may not recommend anything below 85 octane.
One, as already stated, fuel economy will go down. That doesn't matter much to you or I on one leg of a long-distance trip; it will tend to average out and most of us in RV's aren't worried about hypermiling anyway. But if everybody in average sedans runs the low-octane fuel and gets reduced mileage, they'll also burn more tons of hydrocarbons and produce more cumulative emissions -- which messes up the EPA expected calculations that the automakers have to meet. So the warning is to protect the automakers' bottom line, not you.
Second, and related to emissions, running lower octane and at reduced timing can change the overall emission characteristics of the engine. It is possible that some components of the emission control system could have a shorter lifespan if low octane fuel is used continuously over a long time. It may not harm the engine, but it may eventually fail an emission test sooner, resulting in a service bill if you have to replace emissions parts. If it happens under the warranty period and the shop finds out it's due to low octane fuel, they probably won't cover the repair. But putting in a tank or two of 85 instead of 87 on a trip isn't likely to have any significant impact.
Eric & Holly + 2 kitties from New Hampshire
1992 Winnebago Adventurer 32'