Your 37LX has virtually the same floorplan, (but with a larger slide) as my 34E Hurricane. I was curious if your galley sink is plumbed to the black tank for the front 1/2 toilet?
I have not had any issues with the toilets backing up. The front 1/2 toilet in mine might have a different tank configuration because I can't see the bottom of the tank, (I'll admit that I have not really tried looking in there with a flashlight).
I have always used any brand of septic-safe paper in my RVs, and never had a problem. They all dissolve in water just as well as "RV" paper.
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