Seems to be residual water in tub drain but not in sink. I mean if I look down in the tub drain there is water just below the metal cross drain. I can tell it is shallow cause it wiggles around. If I run the water in the tub it drains down just seems to keep a little right there. Is this a shallow trap? some kind of baffle?
Yes, it is a p-trap. The water prevents the odor and gases in the tanks from coming into the bathroom and kitchen area.
I agree. It is there to prevent sewer gas flowing into your living area, same as you have them on every plumbing fixture in your brick and motor home...same reason.
__________________
2011 Toyota Tundra double cab
2015 27RLS
And if you let the gray water get too full - guess where it goes? Yup, the tub.
Wife took us there twice last year while washing dishes/ grandkids were
with us.
__________________
former 2008 Jayco Eagle 29.5 RLS
former 2014 F250 6.7 4X4, CC LB