Looking at RV Trader, the Vista is listing for $74k+ and the Intent for $94k+. In addition, there are none in the Boston area, so you will have to add travel for inspection and purchase, as well as the expense to drive it back home before the trip.
Then you indicate that your wife and kids have time to drive from Boston. Does she feel comfortable driving the rig alone with just the kids?
What if something breaks enroute? Is she prepared to fix it? Does she know enough about the systems to be able to troubleshoot? Or will she be at the mercy of unfamiliar repair shops? (I ask the same questions of you for the time you join up with them).
What about the trip back to Boston? Will you be on that leg or will it just be her and the kids again?
Boston to Minneapolis is about 1400 miles. Driving a large 30+ foot rig is a lot different than driving a car long distance. Is she going to go slow and stop to see sights along the way? Or do you expect that she will do hard driving to get there as quickly as possible? If the kids get bored, tired, argumentative, etc., she will be on her own to deal with that while she's driving - it's a bit easier when you have a spouse in the passenger seat who can focus on the kids.
Is she prepared to cook after a long day's drive? Is she prepared to dump tanks, etc., if needed? Is she prepared to stay in unfamiliar campgrounds without you there?
There's a lot for her to consider if she's going to be alone with the kids for 2+ days on the road each way.