In addition to the manufactured date stamped on the collar, there should be the empty (tare) weight of the tank as well as the full weight. The difference would be the capacity, e.g. 30#... If the supplier fills the tank by weight, they should be setting the scale to the full weight shown on the tank in order to shut off the pump when it reaches that weight. Some rarely see aluminum tanks and occasionally set the scale from memory for what the heavier steel tanks weigh, then the OPD kicks in so at least they can't in theory overfill the tank.
And yes, a full 30# aluminum tank weighs only a few pounds more than a full 20# steel tank. While they still have to be certified, they don't rust and should last a lifetime short of physical damage or theft.
I believe part of the dealer prep on new trailers is to install a large level centered below the front window. It's real handy when pulling up on ramps to know when the trailer is level side to side as it is visible through the rearview mirror over the 30# tanks. I've seen more than one trailer with the 40# tanks and since they come up to bottom of the window with the cover in place, the level is hidden behind them. If the trailer came from the factory with the 40s installed, it would have been better if they mounted the level on the front of the cover well above the jack.
Air Safe Hitch, Reese Dual Cam 800#, Centramatics