We enjoyed August/September. No issues with bugs and it was easier to find RV spots. It also seemed that much of the road work had been completed. Not all, we encountered a number of road repairs in progress but think the volume was less overall.
We may have been fortunate because we did not encounter any snow. We had about 4 days of clear Denali weather. Only four or five days of rain. When we started home the temperatures were dropping down into the high 20's at night but the days were great.
Every year is different so our experience may have been untypical. If we go again I think we would chance the August/September time again.