[Notanlines]

Off hand I would 'guess' at twice a month. I would be way more inclined to invest in a best quality macerator/pump and tie into their system. From Google: "a macerator takes the waste from a WC, a wash basin and a shower then pumps it through 1 inch small-bore pipework, vertically up to 16 feet or horizontally up to 325 feet or a lesser combination of both."
Many better grade RV's come equipped with macerator pumps that may be used if necessary.