I can’t remember the cost of emptying and cleaning the tank but it wasn’t pricey.A few things to remember is that household cleaners and bleaches affect the bacteria within the tank thereby creating smells.If you use natural cleaning products it does not affect it .Our neighbours used to get use something for the toilets and kitchen ,can’t remember what it was,but done the job.White vinegar diluted into a spray cleans most things,neat down the loo.Bicarb and lemon juice in a paste cleans off the limescale.If you have vents in your bathroom or toilet floor pop a toilet block on top.
Boiling water down all sinks and drains regularly keeps them unblocked.
Remember if you haven’t used the villa/apt for a while ,turn your taps and showers on hot ,full power for about 15mins to prevent any nasties like legionella.
Likewise run the AC on full and regularly hoover and wash the filters.