Here are my suggestions or input:
As for traveling in colder weather, I would go north when it's warm and go south when it's cold. People do camp and live in cold weather with an RV, but make it easier on yourself, the wife, the dogs, the three small kids and don't do it. You get into heated water lines, skirting the RV so your sewage lines don't freeze and your furnaces are going to use a ton (several tons) of propane. Even four season RV aren't really designed to live in full time during a cold winter. They're good for a skiing or snowmachining vacation. You really aren't going to do much sightseeing or touristy things (they'll all be closed) at below zero temps.
As for towing a toad behind a MH, there's been a lot of discussion on the forum about that. Most people don't notice it back there and it makes less than a 1/2 MPG differance if that much. Don't let the fuel milage be a deciding factor on towing a toad. Unless you're talking a small MH and a large toad, the mountains aren't going to make it difficult to tow a toad. If you buy a 5th wheel or TT and tow it with a truck, you're going to burn more fuel driving it as a daily driver than you'll use towing a toad behind a MH.
You'll need a generator whether it's built in or carried seperatly. If you buy a truck to pull TT without a generator, then the generator (and extra gas cans) will be in the back of the truck with the dogs.
Maybe you can get a two part shell custom built, but that's going to be expensive. Don't forget to build a heater and air conditioning into the canapy somehow to keep the dogs warm /cool if they're not riding in a nice warm (cooler) MH.
With the kids and dogs, I think you'll be much better off with a MH and toad. If there's a floorplan in a TT or 5th wheel that works, then there has be be one in a MH. Make life easier on yourself. Go with a MH and the weather.
***And get something with a washer and dryer or else you're going to spend a lot of time sitting in a laundramat.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.