slow running drain

Have you checked that the tank vent line is clear?

Hard to completely clog up a kitchen sink drain line to waste tank unless you have been dumping lots of grease and food particles down drain line.
"p" trap clear.......then tank venting

Sorry,it's the kitchen sink. Removed trap and it's clean.

Which drains are running slow?

Sink drain, shower drain, toilet drain...waste tank drain????

Drain lines are same ABS piping used in residential houses.
Waste tanks are also.

Usually just removing 'P' traps and cleaning them out will clear up slow draining on sinks...shower if accessible.

Drain claw will reach down and might clear the clog. If that doesn't work try Power Plumber. Both are available from Ace hardware.

The vent is malfunctioning most likely and easy to replace if you can get to it. I beliee they call it an air admittance valve which would be located under the sink somewhere

Any chemicals designed to clear drains are very nasty stuff. If a tub is slow, it is hair and soap scum. If it is a sink, it is probably just scum. I piece of wire stuck in the drain usually does the trick. A plunger after inserting the wire helps, too. You can always remove the traps under the sinks, but getting to the trap in the tub may be close to impossible.