Welcome! We can relate to your situation...did the same general thing in '03, from California. On the road for 3 years now, and still enjoying it in our 3655FL Montana.
Truck info...get the actual figures, by VIN #, from the dealer as to what the truck can handle. There is a huge amount of info relative to all that on the Forum under "Towing and Tow Vehicles". Remember, towing a 5th wheel is not the same as a towing a trailer for tow rating numbers...Key numbers are the max truck weight, axle weight limitations, tire limitations and combined gross vehicle weight numbers. You will find many folks tow "slightly" overweight, either in the areas of CGVWR and max truck weight. Most everyone really works to stay within the axle limits and the tire limits. Our rig weighs right at 14,000 lbs, and we are OK on the last two items.
Since you are thinking of full time in the Monty, a couple of things that are a bit different for you...first, make sure you let your dealer know your expectations for your rig in terms of both mechanical and cosmetic quality. Be sure to do a really thorough Pre Delivery Inspection (PDI) nd with some of the checklists which you can find on this Forum. Your problem will be that, once you leave your home area, you will have to rely on other dealers around the country for warranty work and other repairs/fixes. Unlike auto dealerships, RV dealers may, or may not, get your rig in for work in a timely manner for warranty work. We have had no big problems as full timers in that area, and have had some work done, here and there. Make sure everything is absolutely working correctly before you leave your home area. If you can, don't transfer $$ until it is that way..that is your "leverage" to get it done, done right, and right now..
Be sure to read the last year's posts in the "Full timers" section of the Forum. In addition, "RichFAA" 's quest for his ""rig" and how it all worked out give a good picture of a really well-researched and implemented "plan" for the purchase of a "full time" rig. Read his posts over the last 6 months of so..very informative! (Well done, Rich and Helen!!)
Again, welcome! Looking forward to crossing paths somewhere in this beautiful Nation.