I really question that the smell is formaldahyde. Usually it is plasticizers that are leaching out of the fabrics and other vinyl furnishings, or the coating used on the wood cabinets.
Regarding the capacity of the 3500 Megacab, I am towing nearly the identical unit as you are and my scale ticket with empty tanks showed a total of 22,760#. My 2006 TV has the 4.10 rear end ratio, so I can go up to 23,000#. However, mine is a 4X2 and your 4X4 will weigh slightly more.
One thing adding to my weight on the TV is the Pullrite Superglide hitch, which I needed when I was pulling a Carriage Cameo LXI and I moved over to the 2006 unit because it was already paid for. One of the other non-sliders should weigh less.
As far as towing is concerned, I hardly know it is back there, except I try not to forget when turning corners in the city or at campgrounds because you have a lot of length back there as well as a full 96" at the back of the Dodge.
Hope you are considering a 2007 Dodge with the 6-speed automatic and the exhaust brake. I wish I would have added the brake on my 2006, but it was a dealer add-on at over $1,500, while on the 2007s it is a factory option for $495 (and that might even get a promotional deduction).
Absolutely love that Megacab! So does everyone who sees it.
Chuck