The thead referenced above does have some helpful suggestions about dealing with the sagging ceiling problem. There is nothing there, however, about how to actually remove the old glue.
In the unlikely event the glue has dried hard enough you could sand it off. Chemically, the best remover is xylene, sometimes sold as xylol. This stuff will do much more than make you high, however. It can make you dead. The only way I would use it inside a closed space like the Airstream is with a full face mask supplied by outside air. Even then I would want a strong fan pulling the fumes out the far end of the coach, and far far away from my air intake source.
If anyone has a better idea I'd like to hear about it as well as I really hate xylene.
'85 Sovereign, 25'