Well, we all know what ASSuming does, but I guess that's all I can do here.<G>
"1-2 cabin lights and the anchor light."
Maybe hat's 3x20W bulbs...60 watts. = 5A
"the sterio installed on the boat"
Similar to a car stereo, could be 5-10 amps draw, let's say 7A.
Most use a 75W power brick, the "desktop replacements" 140W, let's be an optimist and say your's is 75W. = 6A
"Oh, I almost forgot the fan,
I'll guess one amp and hope that's a small fan.<G>
So...that's 18-20A from "moored" till "asleep" five hours? That could be 100AH right there, which is more than a Group27 battery has to give. The assumption is then that you have a hidden nuclear pile on boat to make that battery last throught the night.<G>
Dunno, really, but a typical Group 27 deep cycle has maybe a 70AH capacity and doesn't live long (i.e. maybe only 50 cycles instead of 1000) unless you only cycle it 50% (300-500 cycles) for 35AH of drain between charges. Your best "cheap bang" for the buck, unless you look at real power consumption
, etc., is just to buy two hiking huge 6V batteries to make up a bigger house bank on the third position. Set them up as "B", set up your existing Group27 house battery as "A", and split off the starting battery with the isolator, as the primary battery that gets charged first all the time.
Typically, the wiring
1-Alternator to starting battery directly
2-Starting battery via combiner to battery switch common terminal
3-Shore power set up separately to all three batteries, main battery switch OFF while using it.
I'd do it that way despite a number of flaws (suboptimal charging, etc.) because that's about the only q&d solution. And use the dual-6V- new house bank as your primary bank, because you're probably cycling the Group27 too deeply. When the Group27 dies, replace it with another 2x6V bank.