I live in Idaho and can relate to the colder nights. This year we had hot that transitioned immediately into cold. Having A/C was important for us as we like to travel in summer. It is only useful if you carry a generator or stay in powered sites, but can make a hot night luxurious.
If considering a TC at all I would second the 3500 series truck. For a bit more in cost you increase handling more weight comfortably. Going dually is not required for the lightest campers but the point at which it would make sense is highly subjective and a subject of constant debate here. Knowing your weight numbers is the only sensible approach.
There are dealers near to you in Boise and Idaho Falls, Not sure about Twin. We lived in Pocatello area and bought from Boise, no problem. We bought new on purpose but there are more used TC's available in Idaho than people realize. Very popular here.
Making the TC work for you with a single rear wheel 350/3500 truck is doable if the camper dry weight is kept to less than 2250 lbs. Be sure to have a camper in mind when you make the truck decision. Buying to little truck is a common mistake.
In this area new brands to be had include Northstar, Wolf Creek and Lance. There are others but they become harder to find new. Most RV lots cater to the large trailers. Here in Boise the best chance to walk through new TC's is at Nelson's RV, Bish's, and Camping World. Crossroads in Poky carries Lance. You will need to go to Utah to look at new Adventurer, Camplite, or TravelLite models.
Most of these campers will feature outside showers. It is possible to modify your propane setup to allow for outside grill hookup. You won't find outside kitchens in TC's, but the shower connection and propane mod would give you the basics. Carry a dutch oven for the rest.
No matter what most newcomers say they usually carry more weight than they originally planned. I figured I would be able to keep my Wolf Creek 840 at 3000 lbs. but it comes in at 3250 fully loaded. This puts me close to my tire and wheel limits which is still comfortable to drive with an F350 long bed. Torklift upper and lower spring stiffeners make all the difference.
We evolved from tent camping too, so I thought my wife and I would never use the inside shower. We were sure we would be devotees of the outside shower but that has not proven to be true. The inside unit is so convenient and easy that it gets at least 50 percent of the use. Holding tanks are the only drawback.
For what we do here in Idaho our camper is ideal. If you are interested in more remote, off-road adventure I would recommend the pop-up for better access and lower center of gravity. The Northstars are very nice but heavier and the Four Wheel Campers (Jackson dealership) are really light but pricey. Both were on our short list but we opted for the comfort, warmth and quiet of a hard side.
Having 3 persons sleeping in a lightweight TC is workable. The dinette on our 840 will accommodate a grown adult, but if spending more than a night or two it needs a bit more padding. The upper bunk is only fit for a really small child IMO. For those used to sleeping on the ground it will feel luxurious.
Look carefully at quite a few units before you take the plunge, it will be worth it. There are many floor plans and models to choose from and making the choice to buy used carries its own set of risks. We have been very pleased with the truck camping experience and don't plan any changes until we retire, when we may change to a longer-term solution for extended travel. Even then we might be hard pressed to give up our TC!