Everyone has their own expectations on what they consider enough power. I think the post I quoted below (which is pretty much dead on)should give you an idea of what to expect.
Colo Native wrote:
I came up Vail pass eat bound at 28 MPH towing a Fusion Hybrid thought it was slow but after asking on the forum was told that is normal. FYI I get about 7.5 not towing and about 6.5 towing and I have a 08 Fleetwood 34'
So, do you consider that enough power? Ford owns the RV market today so you don't really have a choice in a gasser. Even the Workhorse chassis that was out a few years back was close to the Ford V10 in power. My dad had the Workhorse and I had the V10 and neither was fast when it came to the grades. I now have a 2014 Journey DP with 340HP and am pushing it to maintain 40mph on those grades so if that is not fast enough for you your going to need a much larger diesel.