I can't speak for Alamo, or for every car rental location, but I do have a friend who works for a Hertz rental location in Northern CA and she says it is in the rental company's best interest to make sure the car seats they rent out are NOT expired, are free from defects, and are clean. She says that they do not want to get sued if someone is in a car accident and a child gets hurt because the rental agency was negligent in renting out a damaged or expired carseat. At her location, they clean each car seat when it comes in, just like they clean the cars. Any car seat reported to be in an accident is disposed of, and they replace car seats a year before the expiration date. She told me that the biggest issue is that they can't install them for their customers (liability again), so she's seen people not put them in correctly. It bugs her a ton, but for legal reasons she isn't allowed to install it, so she can only give them advice and make sure they read the manual. They also don't stock very many car seats because they take a loss on them if they aren't rented out enough times during the life of the seat, so sometimes they don't have one when someone wants it.
Her advice to us when we were traveling was to make sure we were comfortable installing the type of carseat we would be renting from the agency, and to call the specific location we would be renting from to ask about their policy for cleaning and replacing the carseats. She said they should be able to tell us the exact brand and model of car seat they have available and what year they were purchased, although sometimes it takes a couple calls to get the person who knows where to find the paperwork on it since it isn't a question most people ask.