Looking at the wire size charts, 10AWG "should" work, for less than a 30' run. 300W at 34.4V max output from the panels is only a 8.72 amp load, and a 2% voltage drop. Given the limited times that panels actually operate at 100%, in reality the losses are much less... My system typically has my batteries "full", and back on float before noon even on a partly cloudy day, so having it take a bit longer because of line-loss shouldn't be a problem for me.
All that said, I'm still trying to find out what gauge wire Jayco uses for the solar "prep". Drilling the hole in the roof, and running the wire is the "high anxiety" part of installing an RV solar system...