Exterior unheated waste tanks in freezing weather require you to do the non conformist methods.
You must leave your waste valves open, grade your waste line so there is no trap. Always use enough water to carry the waste through the hose. Double flush the commode. Never 'trickle' water to keep your lines from freezing as that means a very slow flow through the waste hose that will freeze there then back up into your waste tanks.
Be certain the camps sewer pipes are below freeze depth so that they do not freeze and back up.
Theres more, but these are proven.