Tip for holiday home owners who don't have keyholders

We have used our holiday home in Sucina 4 or 5 times a year since we bought it in 2009. Every time we were away from it for more than 3 months the water in the toilets had evaporated and the place would stink like a sewerage works until we opened the windows and flushed the toilets. For the last 3 years I have covered the toilets with cling film, this stops the water evaporating. We have just returned after 21 months away (damn COVID), and the toilets still had water in them and the apartment smelled fine.  Hope this helps