You need to also concern yourself with the ratings of the hitch on the RV as well as the GCWR rating of the RV. You should weigh the RV with it loaded for travel and then deduct the weight form the gross combined weight rating and that will tell you how much weight you can safely tow. Power is not the major thing to be concerned with.
Right, safety is the first concern. However, power is an important secondary conern. A small, gas, engine like the 350 will be very unpleasant trying to get up any hills, particularly if you are towing something. My 6L gets 12+ MPG unloaded and 8 MPG while towing. Seeing as the motorhome is pretty heavy (always loaded) on its own, I'm thinking that the difference in gas mileage between a big block and a small block may not be as big as you think. And the extra power will definitely reduce the 'white knuckle' factor while horsing the unit up the hills.