I agree with previous posts. Going to AGM batteries will just make your problem more expensive.
You need to find out why they are not lasting and educate yourself on battery maintenance. A good $100, 220 AH, wet cell deep Cycle battery should last for at least five to six years with simple maintenance. The link below will give you some good bedtime reading on the subject.
https://rvingwithmarkpolk.com/2016/09/21/polks-top-10-rv-battery-tips/
Letting them run dry is one sure way to ruin them. I use an old Ketchup Bottle and a piece of clear tubing to add water to mine. I have a photo of mine but this site is not very conducive to adding photos.
Anyway, it's pretty simple, just take an old plastic Ketchup Bottle (the one that stands upside down). Drill out the hole in the lid to 1/4" and insert a 1/4" clear plastic tube so it reaches the bottom of the bottle and extends 18-20". Fill with Distilled Water, squeeze the bottle to add water to each cell.