Digest can be purchased at Walmart. Thetford is brand name in their RV section. I use the one that 100% biodegradable with no Formaldehyde.
The monitoring panel lights never work that great. They will show full when you just emptied at times. The flicking is the water or what ever running off the sensors. I put some dish washing liquid in the black holding tank when we plan you dr4y camp and travel for a while. It seems to clean the tank somewhat.
Gnats are a pain. We may spray inside of rig with flying insect killer and then plan on being away from the rig for a few hours.