Great pics! This really helps.
The water pump, that is a different brand then I have seen before, but I do not see any issues with it that would be causing your tub draining problem. I still feel you have 2 separate issues.
On the pump, you mentioned you think you have the pump hooked up backwards. There is for sure a "inlet" and "discharge port" on the pump. Most times they have an arrow molded in the pump, or a sticker etc to show which direction the water flow is supposed to go. The inlet is the pump suction port that goes to the fresh tank.
This is the product sheet from Seaflo and I can see the direction arrow molded in the pump in this pic. The discharge port is to the right in that pic. https://seaflousa.com/wp-content/upl...2-Series-1.pdf
Most pumps positive displacement pumps will not pump water hooked up backwards and this brand does not look any different. If water is flowing out of the faucets, the pump is hooked up right as far as the inlet and the discharge.
Also, thanks for that link. These seem to be more in the marine world and I will investigate that brand more as it looks like a good variable speed pump.
The pump you picked is a 3 gallon a minute pump. That is the right size for a camper. You really do not need one any larger, that one will drain the fresh tank fast enough.
To the shower drain issues,
I see you were able to take the cover off the tank vent. And I am assuming that is the "gray water" tank vent and not the black tank vent. Is that correct? There should be 2 on the roof.
Someone has replaced that tank vent cap along the way. It is not the original that Sunline would of used but I see nothing wrong with the cap. Something you can try is to put a long rod or broom handle down the pipe to make sure the pipe is not clogged over at the connection to the tank. Scum buildup of over the vent pipe hole can happen. You do not have to jam it hard, but the broom handle/rod/ stick would stop when it hit the bottom of the tank. Once you know you hit the bottom of the tank, then you know the pipe is clear. Just make sure the stick is long enough to reach the bottom of the tank and still have some sticking out the roof. You do not want the stick to fall in and can't get it out.
Using a mirror and a flash like to look down the hole from the side you can maybe see down the pipe to the bottom of the camper. Or if if have a small camera to look down the hole.
Next is the pipe trap.
This pic (which is upside down) shows the pipe trap or part of it. The U shape fitting that goes down below the floor line and then back up is the trap.
I cannot see the other end of the trap where it attaches to the tub drain fitting. And I'm not sure if there is access to take a pic. If there is, please post one.
That U shaped fitting, is that the pipe you took apart and cleaned?
And you said this. This has me thinking about this and there is a key point to talk about.
Originally Posted by SiaraTaomi
I’ve unscrewed the pipe directly under the shower and have cleaned it several times.
I’m not sure what you mean by pipe trap.
Directly under the pipe leading to the shower drain there is sort of a mesh in the floor. Is this what you are referring to?
Pictures would be very helpful. While using both the kitchen and bath sink they both drain fine but back up in the tub and furthermore, the smell is quite terrible.
You may be cleaning out the pipe trap, which is good but we need to confirm that is what you are cleaning out.
Here is a U tube I found on line that talks about what a drain trap is and how it works. While this is on a house, the shower tarp works the same way.