BIO or non BIO fuel ?

Full bio would be a significant lower mpg rating as it has less power / gal . Bio in the common 5% -20% Mix would not be dramatic, but it is lower no getting around it. The higher % bio should be proportionately cheaper at the pump though as many states give tax advantages to the dealer for pumping it.
As said bio is a good solvent so it cleans well , sometimes to well and clogs older engines filters as it cleans
I ran up to 100% bio years ago in my 2000 Ford 7.3 the engine got about 10-20% worse mileage but the bio was much cheaper at the time. It also ran noticeably quieter!

I see no difference in mileage

We just took a trip, first leg, normal diesel, more extended higher speed runs, fighting lots of wind - 7.6 MPG. We usually run a bit higher but given the conditions I was OK with that.

We stopped and filled with bio-diesel. Second leg, ~ the same number of miles, slower speeds, winds had calmed, no extended higher speed runs - 6.9 MPG.

I was floored at the mileage as since we up sized our tires and air cleaner we had been seeing some decent mileage.

At first I was thinking it may have been a fluke, now I'm wondering if it was the bio-diesel. I believe this is the first time we've ever run it in the motorhome.

Yes, there are several differences:

Slightly less BTU's per gallon of bio vs dino so slightly less MPG.

Shorter storage life for bio.

Bio tends to "scrub" any accumulated gunk from fuel systems, so it can clog filters.

there is in bio-diesel. You don't get quite the boom for your buck with bio diesel i notice about 2 miles to the gallon difference.

no