Talk about a neat score!
Yes - loads, especially lights, will noticeably dim on DC alone, your battery produces 12.7VDC but is charged by 13.5-14.2VDC so loads with AC 'help' grab the cream off the top so to speak.
30 amps over 15-feet of 8AWG (one-way measure but round-trip) will drop 5%, 6AWG will drop 3%. The flexible, fine stranded type cable with crimped lugs is what you want. HERE
is a link to an online wire-size calculator.
Running the battery primaries through a doomsday fuse or circuit breaker near the battery is a good thing. If you find a C/B it also makes a good isolation switch but they don't like to be exercised multiple times daily day in and day out...