Your best bet will be to search for a combination of luggage and each of the stations you want.
Most major stations have self-service luggage lockers that you can use. If you arrive somewhere like Venice at 1100 in the middle of Summer you’ll probably find that the lockers are already full because of all the day trippers who got there before you. On the flip side, if you arrive at 1900 you’ll probably find loads of lockers as all the day trippers are on the way back to wherever they came from.
Most major stations also have left luggage - a manned service where you can also leave luggage. These tend to be more expensive, but in the grand scheme of an Interrail/eurail the difference isn’t much. I’m not sure, but I imagine these don’t ever fill up since there will likely always be *somewhere* in the station buildings that they can over flow to on busy days.
If you do use left luggage, make sure to leave enough time to get your bags back before your train leaves. You don’t want to find yourself behind a load of families or large groups, or whatever who have loads of bags and take ages while you watch the clock count down to your departure time.