The E350 and E450 cut-away chassis with the gas V10 engine and transmission has a very solid marriage. I just don't hear anyone on any forum complaining that their E-Series-V10 overheats or under-performs. And a lot of them tow vehicles including myself.
There are people who just love the idea of owning a diesel. Those who want a mans-man diesel will upgrade into a super-C chassis with a serious diesel engine. If I described you, then look at a "Super-C" chassis class-C. If you really don't care, stick with the Ford with gas V10, tow a typical tow vehicle and you will do fine. If towing something challenging like a quad horse trailer, that's different. Then think about getting a Super-C.
The only issue with the E450 I know of is that it's E-rated tires are an Achilles Heal for the heaviest of E450-based motorhomes. When the rig gets into the 30+ length and filled with people and stuff, the tires get over-loaded.
That long Forester 3271S with queen bed, bunk beds, and huge cab-over bed seems ripe for over-loading. Then comes the tire blow-outs. If only a tire was made to handle the heaviest of loads for the E450, but none exist
That Forester 3271S really should have been built on a Super-C chassis. I found the specs for the 3271S
HERE but even Forest River is shy about specifying it's length. Adding that too much of the house rests solely on the rear axle along with that full-length slide-out, I would be concerned of over-loading the 4 rear tires.
Don't assume Forest River takes into consideration 6 people, a full tank of fresh water, a tow vehicle, and all the rest for their load-safety figures. They do say 12,500 is the dry weight. Do the math.
How much does 55 gallons of gas, 44 gallons of water, 42 pounds propane, your family of 6, food, drink, gear, clothes, tow vehicle tongue weight, etc. etc. etc. You might find yourself driving the motor home, and everyone else following behind in what you hoped was going to be your tow vehicle.