Oh my, 115 Ah would be enough to power 4-5 cabin lights and the anchor
light for a couple evenings. You would have no shortage of power. Without getting too technical:
Assuming 20 Watt lights and 4 hrs of use per day then,
20W / 12 V = 1.66 Amps
1.66 A * 4 hrs = 6.64 Ah per light = ~30 Ah per day for cabin lights.
Since I am sure the wattage differs, feel free to use this to estimate your power needs. The wattage (or amperage) of the lights should be stamped or printed on the base of the bulbs.
The electrics of your boat is something that you should be comfortable with. Find a book or an electrician friend to explain to you what all is happening. Someday something will quit working, and it is generally cheaper to fix it yourself, provided you know the basics. The electrical system
on your boat is relatively simple, and a couple of hours will be sufficient to familiarize yourself with the electrics.