Hello new2motor,
Okay, lets put this to rest. "Horse Power" is what moves you and gets you up the hills. Burning fuel is measured in ˜Pounds' per ˜HP' per ˜Hour'. Not Pounds / Torque / Hour.
As an example; with a ten foot breaker bar, it's fairly easy for me to exert a Torque of 1000 Foot / Pounds, but I couldn't push the RV on level ground much less up a hill because I am only able to produce about ¼ HP at a dead run. (well, not any more)
A jet turbine engine has relatively little Torque but you can't argue about its power because that Torque is produced at a very high RPM, i.e. HP.
If two engines have the same Horse Power but one is produced at 1500 RPM and the other is at 3000 RPM, then the one at higher RPM will be geared down by twice as much to produce a road speed that is the same as the one at 1500. And the gearing will give it the same amount of push to move the vehicle.
The Torque figure for an engine is only used to calculate the Horse Power. That is why the RPM of the maximum Torque is not the same as the RPM of maximum Horse Power.
Horse Power = Torque * ˜RPM' / 5252
The maximum Torque for the CAT C7 RV engine is 860 ft/Lb and occurs at 1450 RPM. But, at that RPM, the HP is only 235. It only produces 730 ft/Lb of Torque at 2400 RPM but low and behold the HP goes up to 350 because of the increase in RPM.
Torque is just a measurement of Force, kind of like PSI. Horse Power is a value of Work and it takes lots of work to move an RV up a hill. Torque by it's self is pretty meaningless unless you know at what RPM it occurs. 1,000,000 ft/Lb at 0 RPM by definition won't move anything! But 200 ft/Lb at 15,000 RPM gets a Formula One race car going at a pretty good clip.
As to what to buy, a Gas engine that produces 350 HP at 4200 RPM or a Diesel that produces 350 HP at 2400 RPM, the choice should not be made on the Torque rating of the respective engines. Rather consider the cost per mile to drive with the number of miles per year and total miles you expect to drive over the life of the vehicle.
For Truckers, the choice is simple, Diesel. A million miles between overhauls, 35% better mileage, and less frequent fuel stops more than offset the initial cost.
In an RV, the question is less clear. The larger the coach, the less MPG you'll get. That alone may make the decision for you. A gas engine costs less to purchase, that's a plus, but you're looking at increased fuel costs, more frequent fuel stops, and if you drive enough, a likely engine overhaul at some point down the road.
And sometimes, it's just comforting to know that big 2000 Lb. CAT or Cummins is back there pushing you!