It would also help to know what kind of load you want to run.
A cell phone might take 10 watts for a full charge, computer about 20 watts per hour. DVD probably also 20 watts. Flat screen TV about 80 - 100 watts per hour.
A 100 amp hour 12 volt battery will store up to 1200 watts, but you already realize that if you discharge 100% then the battery will get used up sooner, and have much shorter life. So figure that it is safe to use about 50% of the total capacity, and still be able to recharge about 1,000 - 1,500 times.
I have a pair of 26 AH batteries in my handicap scooter. They look like they would fit, and cost me about $33 each at a battery store. They are about 6" X 5" X 4". You do not need to be brand specific, or size for that matter.
I guess the remaining problem is how to power your interior lights without running the car battery down to much. LED lights would answer the problem.
AliExpress.com then search for 24 each 5050 LED panels. They each draw about 0.2 amps, and will run hours on a few amp hours. Most will come with a adapter for several types of interior light fixtures.
Good luck,
Fred.
