There numerous full-service gas stations between San Diego and Yuma. Being from San Diego and having retired 14 years ago to Tucson I have made that drive 67 times. As Noted the Golden Acorn Casino is about an hour east of San Diego, you could easily top up there.
At the bottom of the grade west of El Centro, there is a large Chevron station at Ocotillo. El Centro has dozens of gas stations but remember gas in California is well over a dollar gallon higher than Arizona.
Good luck.
:C