Coast to Coast
Well, let's see.
Assuming 3,000 miles each way and 21 days to do it, that averages 300 miles per day, every day. That's a lot of driving with kids. Is there any way to scope down the trip or allow more time. I'd allow 3 months for that trip, not 3 weeks.
One of the reasons for RVing is the leisurely lifestyle, but that pace is going to drive you crazy.
Good luck, nevertheless.