kind of in this situation myself.. have 2 series 24 batteries in my boat. found my alternator was needing replacement (first step) as at idle, it was running battery voltage vs closer to 14V charge rate... i have lots of electronics, radar, 2 charts, lights, downriggers ect.. can be a larger draw... change if not done to LED bulbs if you run these to reduce draw..
Make sure you purchase SAME GROUP batteries, 24s are not very big, i am going bigger here, sure Lithium are wonderful but you can get 3 AGMs for the cost of one and increase your ampacity by different group (27 or even 31 if you have room).
Discussed is a switch for selecting batteries (1/2/both/ off) if you dont have, its a good upgrade ** NOTE** NEVER shift with a motor running, you will fry your electronics connected!
Cover positive terminals and dielectric grease is your friend!
oh, those COSTCO 27 group marine rv batteries are ok (90$ ish) and NAPA makes a 27 group 8301 marine dual purpose ( 150ish$)with a bit more ampacity as well..
if you are running a trolling rig, biggest baddest batteries would be my thought, i know lithium is the go too here but your use/hours really makes up the cost decision .. you can maintain them separate and install a charging system to plug in when home?
hope this helps!
