I had to have my undermount battery trays enlarged via welding to accommodate the Lifeline 220Ah 6V batteries. If that is what you are looking at, be careful; those batteries are 65lbs each and not 56lbs each like the stock ones. You could become a victim of disaster like I was (and one other person here) and have one drop out on the highway. I since had a super beefy new bracket tray setup created and installed on the opposite side underneath because the old area of the frame had become swiss cheese once the rivets pulled out.
This is not designed to be a tell all on the subject, but:
Consider replacing with two lithiums instead... 31lbs each. Yes you have to replace the charging circuit electronics but you will be much happier with 180-200Ah usable instead of 110Ah. And the lithiums can take on a higher charge amperage and not have to taper off the voltage as they get full = shorter charge times. If you use Battleborn, they have freezing temp protection to keep you out of trouble with undermounted batteries. I see you are in Portland. Ask the dealer there about which lithium brand(s) can still fit in your cages.
Yes the 6V Lifelines can take more abuse than standard AGMs... so you can once in a while discharge well below 50% without significant shortening of life, but with the lithiums you can do that every time and they won't mind.