Yes they are a sealed unit.
When I was in Florida the RV mobile repair ''taped into'' the line and recharged it.
That said.
Made my way to Wisconsin a year or so later and needed rv mobile repair to just look at my ac regarding a 'different' problem, 'not' related to refrigerant.
They got up there, saw the altered tap line on the ac refrigerated line and got back down off the roof and said they couldn't work on it.
All the new laws in place about refrigerated units and freon etc. prevents them from working on an altered refrigerated line on an ac.
Might depend on the state you are in how strict these regulations are taken.
I know when one of my house ac units at home had a slow leak. They filled it once and when I called them back the next year they could/would not refill it a second time.