Leisure battery voltage drop help please

Could someone more knowledgable then me please give me some advice. 

2019, swift challenger 645 with solar charger 

serviced by swift dealer in November 2021 and put back into storage as usual 

got message from caravan (yes really! Surprised us too) saying battery voltage had dropped below acceptable levels. Remotely checked and it was showing 9.2v. Drove up to caravan storage (15 miles away), everything looks fine except for two things. 1, solar panel covered in snow/ice and 2. Servicing dealer had left control unit switched on (yes we should have checked it)  

so we’ve cleared away the snow and ice and switched the control unit off. Solar charger indicates it’s charging the battery but I think that will take some days to charge fully this time of year 

so, does anyone wiser than me think this may be the problem and solution? Or is there likely to be something more sinister going on? 

thanks in advance 

G