Has anyone had a plastic tank cleaned. I’ve done it with a steel tank but never have I owned a plastic gas tank
Equipped vehicle (truck)
I am 100 percent positive it got serious mud/ trash in it

Pull it and wash it out with a hose.

Ok. I’ll try that
But it has serious crud in it I think

There really isn't much that "sticks" to those plastic tanks. I'm pretty confident a pressure washer is going to release anything that might be inside (or outside) of that tank.

As a word of caution, be very careful handling the plastic tank, and everything attached to it. be vary careful removing anything from that tank, and even removing and replacing the tank itself. As said before, pretty much nothing sticks to that plastic, that would include any kind of tank patch kit. A cracked plastic tank is a junk plastic tank.

Good point
I might be a little more careful now

Simple green or Super clean

I'm not going to say this is the last word but I think there are at least two styles of plastic pickup tanks. I'm using an underframe tank from a '73 in my '72 W200. It is a smooth plastic and when it cracked I took it to an RV dealership and they plastic welded it. Apparently RV owners forget to drain the water tank and the dealership has a LOT of experience welding tanks. Now the second style I've seen seems to be a more "rubberier" variety that has a hairy surface. I'm not sure what you would do with that style.

After you wash out what you can of the loose crap, throw in some gravel with more cleaning agent and shake around a bunch. I do that with the metal tanks when I clean them. Works real well.