I'm not sure that anyone can answer this question exactly ...
Since you posted this on the Winnebago bulletin board I am guessing that you are asking about a Journey or Meridian ... I am sure that a 300 hp engine and an Allison transmission will get you to the top of any grade on any US highway ... you probably will not be the first one up the hill .. but you will get to the top ... and safely down the other side ...
We had a 36 foot 2000 Itasca Horizon (sister to the Journey with more bells and whistles) with the 300 Cat. We towed a 4 dr, 4WD Tracker all over the west, including the Rocky Mtns. Then a Honda Element for several thousand miles in the east and west. There was adequate power for whatever we needed. We have even passed other RVs while going UP Monteagle mountain in Tenn.
The Cummins 300 ISB02 with turbo has worked well for us. We have run it in the mountains of western North Carolina and Virginia and it has performed flawlessly. It is a great simple diesel, a real workhorse. As said you may not be the first to the top, but you sure won't be the last either. This one has 4800 miles on it and has been averaging over 9 mpg for this trip so far, with 10 plus mpg on certain legs of the run.
"Home is where we park it" SEMPER FI
Winnie 30 RESS UltraLite TT 2015
Ford F250 XLT 2015
I have a 1999 Winnebago Chieftain 36' with 275 hp ISB and allison 6 speed transmission. I tow a 2000 Jeep Grand Cherokee 4X2. The power is adequate for this set up. The six speed transmission is a definite advantage over the 4 speed some units have with the 300 hp ISB. What transmission does the unit you are considering have?