As far as I know, there are no emissions standards on a trailer or truck camper - only vehicles with engines are required to meet US EPA standards for emissions. Any appliance, etc. with emissions would have other requirements at time of manufacture and shouldn't be an issue once they're installed in the camper.
The only requirements you would need to meet is if MT requires you to TITLE your camper - in that case, check with them to see what the requirements are. Trailers are movable and would likely require titling, I suspect a TC won't since it is not a moving object.
If you don't need to title it, then it's just the duty that will be required. It appears that it is about 2.5 percent of the purchase price (looking at the HTS) but better to ask the dealer and be sure. You could do the illegal thing and just load it on and not declare it - but that's your personal ethical decision - I know mine and I would declare it.
