Without weight distribution the 1350 tongue weight will add right around 2000 pounds to rear axle which will likely exceed axle and tire ratings.
The GCWR or GVWR are not necessarily the weak link. You can not go by this alone, or rear axle weight for that matter.
The Axis frame isn’t all that strong at the rear to start with, and it’s already stretched before Thor builds motorhome. Placing a load that far back could stress frame rails too much. I’ve seen truck frames broken in two just above axle.
We’re talking a dynamic load which bounces up and down, creating forces over 2,000 pounds easily. The frame extension can be reinforced but it’s not practical to go forward too far in order to also reinforce the E-450’s chassis rails.
I’m very familiar with this because I reinforced my frame and built my own sub-frame, hitch and receiver setup on E-350. My frame has similar properties but I didn’t have to deal with motorhome frame extension which makes it worse.
I think it helps to visualize things, and 1350 pounds is almost the weight of a couple of large motorcycles. This would most likely overload the rear axle and tires anyway, but even if it didn’t, I personally wouldn’t do it even if within GCWR and GVWR.
My 2 cents .....
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