Buy a bag of washing soda (Sodium Carbonate) from your local supermarket, costs less than one poun for a kilo bag.
In a plastic jug, take about 50 gms (2 ounces) of Washing Soda and dissolve in 1 litre of boiling water. You should wear glasses to stop splashes getting in your eyes, not a dangerous chemical but it's better to be safe.
Then plug the end of the caravan drain and fill with the washing soda solution - leave to soak for 5 or 10 minutes and then flush with clean water.
The washing soda will remove smells, grease, tea, coffee, and soap deposits, just like Mr. Muscle but at a fraction of the price.
Also very good for shower drains, home washing machine, stained tea pots and the like - just wash out with lots of clean water.
Robert