I had lived in the LA area for 16 years and frankly ANY parking close to LAX is tough enough for cars versus finding a decent place for a 43 foot coach.
I am assuming that you will have a car to drive into LAX and park at one of their short term parking lots.
I would forget about considering the Costco parking lot as it will be full of cars. Just take a look at the Google satellite image for Marina Del Ray Costco
Frankly what I would do in your situation is to call the Dockweiler RV Park
to ask them if it would be possible to park your coach for a few hours someplace at their facility or possibly at the Dockweiler Beach parking lot
Dockweiler RV Park phone number - (800) 950-7275
Dr4Film ----- Richard