LOL, this is definitely a subject thats been beat to death many times over the years. :)
There is some truth to what you heard about FOrd's V10. A few years ago, Ford put newer cylinder heads and intake manifold on it, giving it 3 valves per cylinder and resulting in a significant increase in both horsepower and torque. This improvement definitely 'narrowed the gap' some between diesel and gas Motorhomes.
And, as already alluded to, in recent years the EPA finally put emissions requirements on diesels (its about time!), which leveled the playing field even more.
Still, fact is diesel fuel contains more energy in it than gasoline, so as a result diesels can put out more power per gallon of fuel. Soooo, as the power requirements go up, the diesel engine becomes the better choice more and more. Thats why you'll notice as Motorhomes get beyond about 37 or 38' long or over about 24,000 lbs, they are all diesel.
However, all that diesel power comes at a very high price, in several ways (initial cost, maintenance, more weight, etc.)
Bottom line is, you need to look at and test drive both, and pick whichever one has the floorplan that works for you best, that fits in your budget best.
If you decide you want a 40' MH and can afford it, diesel is probably the best choice. OTOH, if 30-35' unit is plenty of room for you, Ford's V10 is very powerful, and is quite capable of handling a MH that size and a toad behind it. A diesel may do so even better, but either will do the job they are designed for and last a long time if properly taken care of.
Oh, and before anyone says 'dont test drive a diesel unless you have your checkbook in hand, you'll never want a gasser then, blah blah blah'....Don't believe it. I personally looked at and test drove both, chose my 36' gasser V10 MH and am very happy with it. No doubt the diesel pusher was a better ride, but $100k worth of better (more than some people spend on a house)!? Not to a working stiff like me, haha. :)