2007 4.3 L v-8... 63,000 miles

Got some rough idling, three misfire codes cyl 5,6 & 8
Also got a P0018 code - camshaft position.

Plugs look good but the camshaft position code has me worried

Is there a chain tensioner solenoid or relay that could be causing the chain to go slack?

How serious is this

 

There is also a P0021 code...

I heard that it may be the chain tension actuator

Anyone have experience with this?