I thought I was going to do something similar. But when I was going through the walk through the day I picked up my camper, I saw that the drain fitting on the side of the trailer was not 1-1/4" or 1-1/2", it was a garden hose type fitting.
Used a similar setup with out pup. Is had a fine mess on the top. Right or wrong.. But when it filled the particles would stay in the container and only the "clean" water would over flow. I eventually made a sewer adapter if we were not boon docking.
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Pat, Jen, Heather & Sapphire, the head mouser.
2015 Chevy HD D-Max
2022 Impression 315MB