Interstate 31-MHD are wet cell "sealed" no battery caps. The guy who told you AGM most likely has it as a hot term and knows nothing of what is really installed. Sounds to me like they are the original reasonable batteries for the coach. At five years old they meet my thoughts on age to replace. I use the five year rule on start batteries " if they hic-up after five years service in any rig I replace"
Interstate 31-MHD Automotive Battery
BCI Group Size: Group Size 31T
Chemistry: Conventional (Wet Cell)
Rating: 12V 117Ah
Cold Cranking Amps: 950
Dimensions (LxWxH): 13 x 6.75 x 9.38 in.
Weight (lbs.): 60
And your $111 price is about normal
LEN