Dennis M M wrote:
Here's some good info to help you get an idea of how things work
The 12 Volt Side of Life
This is a good source to start. Though it's got outdated a little. 19" TV would draw not 120W, but 15 or 20W if you choose wisely. Back then - yes, there was no LED TVs. In essence, 2 batts will barely support you over a long weekend without any 120V devices other than electronics. TVs and laptops have come long way in energy saving.
That battery voltage chart everybody is copying and re-printing without a second thought, while it can be way off if you use AGM batts. Again, there wasn't many AGM users when that page was written.