Our hall bathtub has always drained slow. Now not draining at all. When you take a shower tub will fill and also back shower. When I used washing machine the water came back up through back shower. We have snaked the grey water line. Grey water does not go to our septic system. Only the 2 toilets go to septic. We have found that there is no vent when the bathrooms are. When the sinks and tubs were rerouted to grey off of septic it was vented, toliets are all. Could this be causing our problem. Also at other end of house washing machine not vented up in attic or out of roof just where you put hoses in the pipe. Thanks for any info.

if the tub drains to a hole in the ground and there is no vent, it would be like turning a full 2 liter bottle of coke upsidedown. it will go glug, glug but not drain very well at all.
the water giong into the hole is like blowing up a balloon.

The sinks tubs drain into a 1 1/2 pipe that then connects to a 4 " pipe and is running about 200ft downhill slope into pasture. Where that end is open and clean

Is there a cleanout in the drain pipe, near where it exits the house?

Could there be anything stuck in the pipe maybe half way of 3/4 of the way down the hill slope? Sorry if this doesnt help but im trying here haha.