We are camping for Thanksgiving, and will be smoking a turkey breast on our GMG Davy Crockett.  Can I prep and inject the turkey breast a day or two before smoking, and keep it in the fridge?  Or, do I need to prep it directly before putting it on the smoker?

Should be fine, but I would take the extra precaution of sanitizing your injection if it's homemade. Just bring it up to a simmer for a bit and then fully cool before injecting. Also sanitize your injector.