You have to go with your gut on this one. Yes Big bend is remote and adds lots of miles to a trip but we were glad we done it. We covered about 3800 miles in 18 days which was a lot of driving and not something I would have done with 2 young ones. However if you are to cover around 3000 miles in 3 weeks (or more) on the road it does bring your average daily mileage down. Of course the deeper you probe into the other places you are visiting the more alternative places you will find while covering less miles.As I assume travel time will be slower with an RV (which is OK), my wife is strongly suggesting to drop the Big Bend from the itinerary. I would hate to drop it, but I think she is right.
The RV does seem a good deal but make sure you get all the fine detail such as insurance coverage and any other charges that will (or may) occur. When doing your costings remember that the RV will only do around 9mpg and there will be campground fees.