Hi and welcome to the forum!
Originally Posted by Martino78
Next time you drive your rv flush a bag of ice in your toilet. While you are driving around the ice will break down and scrape up any hard "substances" in your tank.
This is a tried and true method I've used a number of times. Just remember to first flush you tank and then refill it with fresh water to at least about half full.
Also, if you can take the time, you might try to fill the tank with fresh water and add an enzyme cleaner designed for septic tanks. It has to sit for a week or so to be effective.