Welcome to the forum! Congrats on your purchase. We have really enjoyed our 25.2.
Did you check the water in the chassis battery? The original batteries, both chassis and coach, in my 2016 Vegas are still doing fine, but I have been careful to keep the water up and never let them completely discharge.
Any auto parts place can match your battery for a like replacement. You can lift a top post battery with a pair of vise grips, or many of today's batteries have a built in strap.
I can't help you with the AGM question, someone else here will have to chime in.
You'll find a lot of useful information in the excellent crowd-sourced manual that forum guru Ed Felker put together. So superior to the generic manual provided by Thor.
Here's the link: https://1drv.ms/w/s!AiYx6DLSeiiP53x246awlGrg8B-I