i would buy a new rocker either you or someone else put it in get the body work done than bed line the rockers up to the trim piece on the door or the body line than id pull my inner fenderwels clean and undercoat the inner fender all the way around id adress the door rust now before it gets any worse

 

Bed liner is ok for stopping rust on the outside but it'll still rust on the inside. happened on my truck.

 

I thought of removing the rubber plugs on the inside rockers and spraying used motor oil in there with a sprayer. Any thoughts on this?

 

I don't know that I'm going to do anything about it. Might end up just getting a newer truck in a few years time. We bought it because we bought a boat and needed something to pull it ASAP since we found out our SUV couldn't pull it.

I'd really like a 3rd Gen 5.9 Cummins with an auto.

 

There is weld thru primer with zinc in it to inhibit rust. some people say its the cats a** but I'm kinda on the fence with this stuff.
Another thing you can do is after the rocker panel is on is spray a good inhibiting paint in the rockers by taking the rubber grommets out of the back side of the rocker. make sure you replace the grommet after though.

 

If that primer is like cold galvanization it would work in theory. For those of you who don't know galvanization works because the sacrificial anode will corrode before the material you are protecting does. In this case the zinc will oxidize before the steel does. Therefore no steel will rust before the zinc is totally gone (or out of electrical contact with the steel).