2015 Winnebago Forza 34T, 340 HP enough?

The only improvement you have made is the addition of the turbo which is able to compensate for the high altitude. At high altitudes, your DP will perform like your gasser did at low altitudes. So if you gasser was able to handle the grades in low altitude, then you DP would be able to handle the same % grades at any altitude. If you were unhappy with your gasser at low altitude, then you will be unhappy with your DP at any altitude.