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.
Pat, Jen & Heather... Oh yeah and our trained attack trailer cat, Cinnamon
2015 ChevyHD D-Max
2014 Greywolf 26DBH