If your battery tests out fine, you have a parasitic draw. Get an amp meter and find out what the parasitic draw is to start.
1.5W is next to nothing. If you have a battery with no parasitic draws, it might replace the natural loss of charge or at least give you a longer period before it becomes an issue.
As someone mentioned, something as small as a glovebox light might be drawing 10-15W and unlike your solar panel, it's drawing 24/7.
While a 200W panel on the camper should compensate, it's best to fix the underlying problem as it could be a sign of something else that will cause other issues if left unfixed.
