Why or how would water back up into tub when not used?
 

okay, so i finally got my cut off installed for the line going to my toilet and was able to turn the water on to the camper. the sinks both work. Well after i ran the water a little bit trying to get the air out of the lines i noticed the tub drain pipe had water in it. the tub is actually taken out of the camper right now but there was water all way almost to the top of the pipe.. i didn't run any water in it. I thought maybe the water backed up into that line from running the bathroom sink?
Well i ran both sinks some more to see if it come up higher it never did. i ended up pulling the valves on the camper and just water came out but the pipe was still full of water. So what would make the shower drain have water in it and why wouldn't it drain when the exit is opened up...
I will say that side of the camper feels it might be a little lower then the other side but i would still think it would drain

Thanks for any info.

Just guessing, P trap in the drain. Without it you wouldn't be able to stand the smell in the camper :)

Sounds a good guess to me, Gary!

Even grey water smells bad after a while.

- Jack

It is in the P trap. it was completely full are you saying it should always have that water in it?
I had used a shop vac and sucked the water out of it when i took the pic but it was about a inch away from top of pipe where the shower drain screws onto it at.

Yup, if there's something in the tank, the P-trap should also be full.

I suspect you may get occasional freezing temps where you live. If so, you need to make sure ALL the P-traps are either "dry" or have RV antifreeze in them during freezing months.

You'll have P-traps under the sinks too.

- Jack

What Jack said :)

Okay, I didn't expect there to be that much standing water. I filled the bottom of my 5 gallon wet vac.
Yes it gets cold here in winter. in the 30s 20s.. I never had issues with my inside plumbing on my old Camper i lived in but the outside pipes going to the sewer at the RV park i lived in i did have issues with it freezing. I plane to make sure everything is nice and insulated before winter..