The least expensive way to go is to buy a used truck and a used 5th wheel trailer. But as mentioned before they are best suited for parking in one spot and using the tow vehicle to drive around an area.
Trailers, whether a 5th wheel or a conventional trailer will provide the most interior space and carrying capacity and sleeping room. Want to carry bicycles and kayaks or an ATV then a conventional trailer leaves the bed of the truck for safely carrying this toys around.
With a motorhome unless it is a Class B it will be too long to park just anywhere and you will have problems with most Class A type in many US Forest Service and BLM campgrounds.
Many places where we travel there are no places to pull over as in Yellowstone, with a trailer or with a motorhome longer in total than about 25 feet. Going to Zion or Arches the problems are similar. Stopping for groceries or fuel is a lot more difficult with a trailer or a motorhome towing a toad.
I strongly encourage you to take the time to visit campgrounds within a days drive of your house and go talk to people with different RV's and see how they handle storage and getting setup and what they like about their setup and what they don't like.
Two sets of friends recently made changes to their RV, one having gone from a 28' 5th wheel to a 35' 5th wheel and now using a 30' motorhome towing a car. The other couple downsized to a Class B and after their first trip had it modified to tow a small 4x4 vehicle.
At the campgrounds you can go inside and see the setup as it is being used and get a much better idea as to what might work well for you and what might be a problem.
