Not everything Thor does makes sense. However, in the past, you have not been able to rely on the info on the Thor site. I am not sure they ever changed the 24.1 from a E-350 to a 450 in the literature. My Vegas E-350 was always right at the max weight for the chassis when I weighted it. My E-450 Axis is 2000 lbs light in the rear. I like that. I would not want to go back to a 350.
You're probably right on Thor not previously listing or making reference to any Axis/Vegas being on the E-450 in marketing literature (as far as I recall). That's what makes this strange -- that they show 4 of the 5 floor plans as E-450 but the 24.1 as E-350.
It could be an error, or perhaps a revision as they switch to 2018 model year.
Yeah, the E-350 at up to 12,500 pounds GVWR is a great size between the smaller Transits, Sprinters, and ProMasters, and the heavier-duty E-450 which goes up to 14,500 pounds.
There are so many Sprinter-based motorhomes on 11,030-pound chassis that I don't see why the E-350 can't be used very successfully. I'd personally like to see greater use of single-rear-wheel E-350s to compete with ProMasters and Transits.
Axis and Vegas 24.1 floorplans will indeed be manufactured on E-350 chassis for the time being according to rep I spoke with. Other floorplans, which are longer and heavier, will remain on E-450. He said once supply of E-350s runs out, they'll likely go back to E-450s.
For those who may prefer the E-350 this may be an opportunity. Those who only want the E-450 now need to avoid getting an E-350 unknowingly.