Good luck on figuring out why the tank lights are inaccurate. They are notorious for being near useless. On my Roadtrek, they have never been reliable, even when I know the tank is empty. Iíve learned to never add fresh water without also dumping is the way to go.
Your macerator pump probably looks something like the drawing at https://www.pentair.com/content/dam/...ne/911-566.pdf
. This document says MAINTENANCE TIP!
Loosen stuck impeller by turning motor shaft clockwise from rear with a flat- tipped screwdriver.
Sooner or later the impeller (part #9) will go. When mine wore out, I had it replaced and kept the old one and got the rebuild kit and now keep it around as a spare.
Engaging the knob requires pushing it in very hard on my Roadtrek as it is a somewhat flexible shaft and rotating until it engages the screw slot. Look underneath your Roadtrek and see if the knob shaft is really engaging the flat bladed screw slot. There should be some resistance.