Carol from Halifax, Nova Scotia writes: "I have two toilets in a vacant condo that haven't been flushed for a long time and now are completely dry. What will happen if I use buckets of water to fill them (until the level is back in the bowls) and then fill the tanks the same way? I am afraid that there will be a pocket of air somewhere and when I flush them I could damage the pipes."
There will be no "pockets of air" to give you any problems. Whether you fill the tank and the bowl or just the bowl, there is no place in the system to trap air. Filling the bowl can be critical to avoiding sewage odours from coming back up. Filling the tank is not necessary, simply leave it dry until you want to use the toilet.
If there is any chance of freezing temperatures, you should fill the bowl with RV Anti-Freeze liquid. This will prevent ice from cracking the bowl, but is specifically not harmful to septic and sewage systems. Another trick is to add a little oil to cover the top of the water and that will radically slow down the evaporation out of the bowl.