Scott. I just did this. Get the wet cells, They work just as well and are less than half the price. If your 270 is factory wired each battery is connected independently to a 2 bank charger. The shore charger is independent from the battery switch. You can leave the batteries off and they will still charge from the shore charger. What everyone else said is also correct. My opinion is to get 2 deep cycle wet cells and use them in "both" mode They will easily get you through a night at anchor with lights and music running. I invested $50 on a charger pack from harbor freight. I never had to use it but it is worth its weight in gold for the peace of mind it provides. Just charge it monthly. I have group 24 size batteries. I would check to see if group 27s fit before buying them. The batteries are in pretty tight. Also NEVER switch off the batteries or the ignition switch while the engine is running. Apologies if you already knew this.
