Snapper Riding Lawn Mower Battery Keeps Dying

I have a Snapper Classic 15.5 HP (Briggs) 33 inch Rear Engine Riding Mower. For the last few months my battery keeps dying on me. The previous battery I had was working alright around June 2022, then it started acting up and come October/November it completely died. Batteries + said it was so dead they couldn't even charge it, figured it was time to replace anyways and got a new one. Had no problems with the new battery on its first go, but every time after that I had to leave it connected to a battery tender after each use. Now not even the battery tender will keep it going and I have to jump start the battery or or use the pull to get the mower going. Just used it today and had to pull to start and it struggled the whole time. 

It can't be the battery because I only got it maybe end November/early December. I suspect something is draining the battery, but I don't know where to start troubleshooting in order to pin point the problem. Best I can figure from my internet research is it is either the starter or the alternator. Any help is appreciated, thank you!