Did you buy your rig new? If not, there can be a lot of sewage gunk on the walls of your tanks. The GEO method recommends 1 cup of powdered laundry detergent diluted in hot water. Add to toilet. If you are not using the gray tank a lot, add the same amount down your drains (portion it among the drains). Then add 2 cups of Calgon water (not fabric) softener in the toilet and also 2 cups down the drain. Fill both tanks with water. Let sit to loosen the gunk. Drive around for agitation action. Then dump. You may have to repeat several times to get tanks clean. Continue same additions thereafter each time you dump the tanks.
Usually you wait till your tanks are 3/4 full but if you rarely use your tanks, then you need to add a lot of water before you dump.
Not sure why Beth said not to put bleach in your black tank. Bleach kills bacteria and a bleach dilution could help. I just would not leave it sit. Add, drive for sloshing action and dump.