Grants is about an hour farther East than Gallup, but I think there may be more
things of interest to do. In addition to those mentioned in the Trip Advisor link,
El Morro National Monument and
Acoma Sky City are not that far away.
In Amarillo,
The Big Texan and
Palo Duro Canyon are popular attractions.
If your primary goal in Memphis is to see Graceland, the
Graceland RV Park is as convenient as you can get. It is right behind the Heart Break Hotel and a short walk to the attractions. That said it is right in the middle of a heavily developed area and Tom Sawyer may be better for the kids.
In Sevierville, there are two other parks (Riverside and Two Rivers) that like Ripplin Waters are North of the major development of the towns of Sevierville and Pigeon Forge. I would recommend staying at one of these three, if possible, to avoid towing through all the traffic. Parking in Gatlinburg is expensive and a mess. There is a
trolley from the first visitor center on the North side and starting June 13 a
free shuttle.
Hope you have a great trip!!!