Make careful notes about all the appliances, A/V equipment and HVAC systems in your vacation rental. Model and serial numbers are a must. But also take measurements. If the fridge breaks and you find yourself ordering a new one to be delivered, you don't want to have to ask your already inconvenienced guests to measure so you can order the right size. Lastly, take photos. Lots of photos. If you don't then I can guarantee you'll get into a conversation like this with your repairman "Oh sure, I know that model. So do you have the version with the pipe on the left or the right?"
When you're visiting your vacation rental, ask around for recommendations of local handy-man and repair services. Having all this information available will greatly smooth your ability to address problems when you get that dreaded call from your guests that the A/C has stopped working.
Also seriously consider getting an annual service contract for things like HVAC. Not only will annual service keep the systems well-maintained and less prone to problems, but having an established relationship with a local repair company will help you get to the top of the list for attention if something does go wrong. If, like us, you decide the annual fee for a service contract is worth the expense for the peace of mind, then consider also including their phone number in your guest information book.