If the 5th wheel is GVWR of 12,000 you are fine, the air bags would help support the extra 300 lbs of hitch weight without overloading your truck OEM supplied suspension. I would comfortably pull this combination.
My opinion but I would check your local road rules/regs to ensure you are within the laws of the road in your area; I am not familiar with CA rules and level of surveillance and enforcement.
Inflate the air bags so the truck/trailer are level when hitched and loaded, wet with max cargo (ie full tank of fuel, water, and heading off on a two week holiday). My experiences are airbags need about 25 to 40 psi.
I carry a bicycle pump to adjust air bags while on the road - ie to add air.
Hope this helps.
Take care,
Brian