There shouldn't be any problems with anti-freeze not working over time, the ethanol or propylene glycol won't change into water and break your pipes. If it was winterized properly you should be fine indefinitely. There are plenty of other things that cause problems in storage though, especially the ones with tails
If you're worried, you could just drain it out at the low point. Any areas that don't drain completely will still have anti-freeze in them.
Hope your health improves soon and you can get out on the road again. For my money the best anti-freeze is taking the trailer to a place that stays warm in the winter