[2022] Is the Ambulance a Safe Storage?

To answer the OP directly, Yes, it's technically safe storage. There is the risk of zombies breaking it, but, it's a relatively minor risk. And while loot can respawn, it will NOT respawn any container that currently has loot in it until that loot has been removed.

So, all in all, it should be about as safe (if not safer due to the massive HP of the ambulance) as just plopping your own storage chests down.