Do you have a trickle charger on your Towing vehicle ? if you owned a motorbike did you trickle charge it all week or until you decided to ride it ?
A battery will stay charged and have no problems even if you leave it for a month or more.
I have a work Van that sits next to my house that I use maybe once a month or less as I retired a few years ago and the Van comes in handy now and again

and yet I can jump into it and start it first kick every time.
You only need to charge a battery if it starts to go flat, if you ride it once a week then all is good.
If you Winterise your Ski does the battery go flat, no it won't unless it's getting near its end date

A Ski battery is no different to any other, if there is no draw then it stays charged.
If you are worried about the battery try and start the Ski the day before going out and if it sounds sluggish then chuck it on charge, make sure you disconnect the positive terminal before charging.