The motorhome is undoubtedly more conveinent for the actual traveling part; you have more ready access to snacks, the potty, travel activities, etc. There's also just generally a little more elbow room for the passengers.
I'd really suggest staying the overnights at some sort of campground, rather than planning on using rest areas. The nights will be more restful when you don't have trucks coming and going and idling next to you, and you'll have access to hookups and/or a showerhouse, and some space to stretch the legs and maybe a swiming pool. Many states also have time restrictions on how long you can stay at a rest area (which are not always enforced), meaning that to be technically in the right you may be obligated to move halfway through the night. If it were me, I'd also probably plan an additional travel day, but of course you know your limits and what your family is comfortable with far better than I do.
