If it were me, I'd pull the battery and take it to a shop and have it tested (unless you can do it yourself). A good battery fully charged at rest measures at 12.5 volts. A bad battery at full charge at rest no load is at 12 volts. A good alternator attached to a good battery measures 13.2 volts to 14 volts with the engine running.
Secondly, storage temp in cold weather can also contribute to decline as well as state of charge when initially stored, eg., was it subjected to short stop-&-go before shut down?
Lastly, buy one of these and use it, I've been using them for years, they work great:
Amazon.com: Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger: Automotive