Why not use the truck for your camping/racing needs?
If you do decide to buy a generator, do the math first. The SR battery has 98 kWh, the ER 131. How big is your generator? Is it 110v or 220v? How fast can it charge the truck 10%, 25?, 50%? Does a generator even make sense for your need? Are you boondocking or can you charge at the campground?