Someone at OEV told me yesterday that a 350/3500 would work.
350/3500 will work for base High country, but it is borderline. When heavy options (such as the oversized cabover, air conditioner, etc.) are added, OEV recommends moving up a chassis. So it really is about how you option the camper (base trim vs add all the options) and how you pack. Lots of recovery gear, towing a trailer (tongue weight), lots of passengers/gear/toys with, and you'll quickly be over max payload on the 350/3500. I think this will be similar to the discussion between using a Fullsize (aka 1/2 ton) vs a HD (aka 250/2500/350/3500) for the smaller slide-in pickup campers. When wet weight has you on the border of being overloaded, it greatly depends on how you pack, what you bring, aftermarket overlanding pickup mods (more weight), if you tow or have receive carry rack (storage, bikes, motorcycles), how you drive, etc. Driving a rig that is well under max payload us just much more relaxing and enjoyable than being right up against max, but each owner will have to realistically evaluate how they pack, drive, etc. and decide.