If neither the charger or the battery are connected to the vehicle, the vehicle cannot possibly be damaged by the charger.
The vehicle will forget the time, and the radio may need the code putting in again.
If the battery was running down, the current was going somewhere. It could have been to the vehicle, to the solar charger, or internally within the battery.
If you disconnect the vehicle, you have removed a possible discharge path. The solar charger should incorporate a diode so that it can't discharge the battery. However, a small solar panel, in the winter, won't do a lot.
I suggest that you test what the solar panel is doing by putting a milli-ammeter in series. Most multimeters will read milliamps. You should get no current with the solar panel in darkeness. Any current with the solar panel covered means that the solar panel is discharging the battery.
In sunlight, you should get a few 10s of milliamps charging the battery, which should be enough to keep it charged.
However, if the vehicle was taking a few 10s of milliamps all though the winter, that will flatten the battery, and a small solar panel won't make much difference. If you have now disconnected it, then a good battery should hold its charge well enough that you don't need to keep it topped up over winter.