A agree that chemicals other than plain old water are not needed on a regular basis. Prior to the last trip of the season home, I fill the tank about half full & pour a couple of cups of low suds liquid laundry detergent & then dump & rinse close to home.
As for not breaking down solids, the "technician" clearly doesn't know what he's talking about. I have a clear elbow on my hose & the only thing I've seen that's remotely solid is the occasional shred of TP. Poop breaks down very quickly just by sitting in the water for a few hours.