350 HP ICE vs. 350 HP Diesel?

There is much more difference between the coaches than just the engine. Most diesel coaches have rear engines. That means that all the noise, vibration, and heat are 30-40 feet behind you, instead of right between the front seats. Diesels have air suspensions that perform much better and smoother than the spring suspensions on a gasser. Diesels have more carrying capacity and get better fuel mileage and range (although not better fuel economy because diesel fuel costs more than gasoline). There are many good reasons why a diesel coach costs so much more to buy. Drive them both and you will see. The diesel ride is quieter, smoother and more controlled. Most diesels have larger fuel, water and waste tanks, and more amenities.

This is not to say that a gas coach will not do the job, it will. But it is like asking if you want to drive across the country in a Chevy or a Mercedes.

No question-- HP is a HP. But, how many are willing to run their gasoline engines at peak HP RPM for long periods? Diesels, no problem!

As Doug said, with proper gearing, acceleration and grade climbing is all about HP, HP and HP.

DougE wrote:
Quarter mile performance is mainly a horsepower and gearing thing. I wish they had done a high gear only hill climb up a long 5% or better grade.

The loaded trailer quarter mile test equals a hill. A load is a load, whether it is gravity or a heavy trailer.
The same diesel engine could probably pull 10,000 pounds more than the gasser but the test just shows that all things being EQUAL, the gasser will out perform the diesel.
When the coaches get above 26,000 pounds in weight, the gas engines are usually out of the picture so it all depends on whether or not the floor plan that works for you is available with both engines.

Quarter mile performance is mainly a horsepower and gearing thing. I wish they had done a high gear only hill climb up a long 5% or better grade.

In the Feb 2014 Motor Trend magazine they gave road test info on two Dodge pick-ups. One with the hemi gas engine and one with the Cummings diesel.
The hemi was 410HP and 429 lb-ft torque. The Cummings diesel was 385 HP and 850 lb-ft torque. Basically the diesel had twice the torque as the Hemi. So how did they compare?
Quarter mile empty acceleration 16.5sec @86mph for the hemi and 17.1sec @82mpg for the diesel.
Yes, but with twice the torque, the diesel has to have the advantage when under a load, right? They then did the quarter mile while towing. The results were:
22.5sec @ 64.2mph for the hemi. The diesel came in at 26sec @ 54.5mph or significantly slower.
Now the load rating for the diesel is such that it can tow a lot more than the gas engine pick-up, but given similar loads, the gas engine performed better, in spite of having 1/2 the torque.
As one that has had both, all I will say is that both engines will get your coach and toad to the top of the hill. There will always be the usual arguments about which one will last longer, but the instances where an RV owner has worn out the engine, before the coach, is non-existant. Both engines can and do fail from lack of maintenance, failed components, and from poor operating practices. Not from mileage.
Buy the coach that has the floor plan that works the best for you and don't agonize over the engine.

Not sure of the question-- diesels ARE internal combustion engines (as are diesels).

And it is not possible to "assume all other variables are equal"-- a 350 HP diesel engine will have around TWICE the torque of a gas engine of the same HP.

There is a reason that both engines are offered in RV's-- and often the question sparks "Ford vs Chevy" support for a persons preference. You can do a search for gas vs diesel and come up when many such discussions.

In a word: TORQUE!!