I entirely understand your issue - many of us find ourselves in the same boat. We have a 2006 Suburban with the 8.1 that we use for towing and local hauling and that's it. We bought it used almost 7 years ago and hope to get at least another 3-4 summers out of it. Our oldest is a bit older (she's 13 now), so we figure that will be the tail end of our longer camping trips with our full family of 5.
All of this is to say that there simply aren't good options for circumstances like ours. I would suggest that your best bet might be to reconsider whether or not you need to camp on all of these western trips. You are talking about a massive financial investment to replace that vehicle with anything that would suit your needs, and it won't suit it as well as your current vehicle does.
Give it a try and see what happens. If it works, great. If not, you can do all of your non-mountain trips with your current set-up, and take a different approach to the western trips you might make. You could leave the camper behind and use hotels or campground cabins at what would likely be a much lower total cost than what it would take to upgrade your entire set-up.