1) Mine was sensitive to ethanol gasoline over one month old. Symptoms are like old vapor lock. I use only non-ethanol marine gas; problem gone. At least on other RTV500 owner has reported identical problem here.
2) Suspension is set pretty stiff from the factory. I adjusted mine from center position to lower position and it rides more comfortably. (five increments available.) Manual says you need a special spanner, which Kubota/USA does NOT sell. Adjustment made easily with a large "Channel Lock" plier.
3) After a year RTV500s sag to the left, driver's side. Stiffen left spring one notch to even up. Kubota cannot provide one spring rate for drivers 70 pounds to 315 pounds.
4) Great paired with a MUTS all-terrain, dumping trailer.
I think it had less than an hour on it when purchased new. Now up to around 8hrs. Most trips are only a couple minutes. Worked awesome to spray some weeds on Saturday.
In Florida we have year around high humidity and warm temperatures, distinct from your Upper Mid West conditions. I have been told that our ethanol never gets a "winter" blend, we get the same gas blend all year. It is possible this is more a regional SE USA problem. I do not know.
Perhaps others will share their RTV500 fuel experiences.