Plugged Drain

Try using a snake if possible.
I assume you drained the tank.

simple thought. did you open the gray water? you should not have a plugged drain.

That usually happens when the holding tank is full. Any chance on that being the case?

For the best and safest drain cleaner is Vinegar and Baking Soda non toxic, safe and works better than any drain cleaner I have ever used.

Been using it for years

Water coming up shower my first thought is our gray tank is full.
Only done it once. Id start with something a bit less intrusive like a plunger in shower, generally last in line before holding tank (towel over kitchen drains). Ideally hose connect and at dump station.
Does it gradually drain?

If you're super picky about never putting any kind of dish washing water, grease, etc. down sink drain there shouldn't be a clog. If you use it be real careful with the Draino and really rinse well after. I keep a wire drain screen in all sinks to prevent anyone from inadvertently dropping something down the drain. When we were testing things at our son's house where we bought the camper water came up in the shower pan too. Duh, we didn't read the little note previous owners left that said to always open the drain cap below the black water gate valve or it would back up.....no on board grey water tank! LOL live and learn. Blue boy tank a constant companion now as no space to add a grey tank on our old unit.

when he turned the handle outside the camper the water came out so that's where im stumped water is going into the grey tank but some up through the shower drain. unless when you pull the handle the line blocker thingy ( I don't know what it called LOL) doesn't close or open all the way when you push it in or pull out. Sorry I hope this makes sense
The water comes out at a regular flow as far as we can tell.

At the small compartment door on the left in the right corner of it is where the grey water handle is we turn it open and water does come out but also in the shower.???

As the water fills the tank it can overfill. At that point it has no place to go other than back out a drain. That's why you have to pull out the 'dump valve' in order to drain the tank.

You have me a little confused. You say you turn the grey water handle to open it. All of the the dump valves I have seen are pull to open, and push to close. You can turn the handle, but it does nothing.

The shower drain on my TC also backs-up and gray water flows into the shower pan when the tank is full.

Wayne