Also, when parking in the infield, you dont get to see much of the race, unless you get up on top of your RV. Las Vegas has RV parking overlooking the track, but I wasnt able to find the cost on their website, but I would guess between $500 to $1000 for the weekend, if you can get a ticket. Phoenix is putting in RV parking overlooking the track, and I believe it will be done by next year. I would guess the pricing would be about the same as Vegas.
If you are planning on sitting in the grandstand at Vegas, you need to get up about 30 rows in order to see over the media building to see the back stretch.
In Phoenix you dont have the problem with the media center, unless they are building one during their current remodel project. Also, at Phoenix if you dont park on the overlook, the RV parking area spreads out to the west of the track, but they do have a very good tram system, so you dont have to walk too far from your RV to get on one.
Hope this helps.