I try to add 5 gallons of water, borax, and zep concentrate to both tanks, then drive to our destination camp ground. Works really well for smell and sensors. I've used the "swizzle stick" from Walmart as well. I think the reason both worked for us is because the paper on the sensors didn't have a chance to turn to cement. The measurements for the borax are on you tube. Look for the "poo pyramid" from the rv guy. Looks like a 1960's Berkeley resident but really knows his stuff.
https://www.trbowlin.com/bio-geo-method/