I can't promise that this will work for everyone but, since October 2020, we've been full time RV'ers and this works for us. YMMV.
It's a multiday process. Never takes more than a few minutes on any one day.
For us, tank dump day is generally Sunday.
Sunday (post dumping) - 2 gal of water with 1/2 cup of Borax (mix well since it's powered and you want it all to dissolve. great time to scrub out the toilet bowl since borax is great at cleaning, softens the water and helps break surface tension)
Monday - Flush one of these pods
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I follow that by taking 1 packet of yeast (from the baking aisle) with a pinch of sugar dissolved in a cup full of warm water to make them activate. Once the yeast get all bubbly, into the toilet and down it goes. Yeast feeds the beneficial bacteria that I just added.
NOTE: The composition of septic bacteria products is different and usually involves anaerobic bacteria which, as a rule, makes a big stink. I try to foster aerobic (and less stinky) bacteria in my tanks.
Tuesday - 1/4 C of Dawn and 1 gal of water (helps break surface tension and get rid of greasy residue - also a chance to scrub out the toilet bowl)
Wed-Thurs-Fri-Sat drive around or crank up the music and have a dance party while cleaning the rest of the RV to agitate the tanks
I have found that this method keeps them from stinking even though the Dawn soap is the only scented product on the list. It seems to keep both the black and grey water tanks clean and odor free. The grey tanks have a modified schedule and plan. We generally treat the grey tanks once a month (not weekly) and only use enough water to dissolve the borax and dawn soap. We rotate which drain (kitchen, bath sink, shower) gets the pod.
When we got the RV, the sensors had quit working and we had a definite poo pyramid going. Our in-laws had repeatedly pulled into RV parks and just kept the valves open (which is a no-no) followed by parking the RV for extended periods of time with sludge in the tank (another no-no). So it's so different from the OP's problem with the ledge. It's taken a few months, but this seems to have started to resolve the issues we were seeing. The sensors are working again and we're only dumping once a week vs dumping every 3 days or so.
Using the tank washers doesn't hurt either. Some RV parks will rent you the tank rinse wands . I haven't used them but I've talked to several people who swear by them for cleaning out that list bit of sludge.
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