If it is a Coleman unit, it is probably a sealed unit without Schrader valves. It is more than likely R22. If it is low on refrigerant, it is leaking and that needs to be fixed.
It is possible to retrofit the schrader valves, evacuate, repair and recharge, but you'd have to be able to do it yourself. It can be done. You'd have to decide if it is worth the effort or not.
I would suggest a new Dometic Penguin II low profile high capacity unit.
Work is never done, so take time to play!