Mine are at 10 years and seem to be holding up well. Hard to compare because when new they were powering a plasma energy hot TV and halogen lights... now all LED. This past six month we did a lot of dry camping... probably 50 days with four 5 day stays. I'm now residential but have more AGM batteries for it than it really needs. Two hours of generator time kept them up which is less than when new BUT we used more power when they were new.
I would do a test by turning off the shore power and living in your rig for a day or two and see how it goes. Instead of cranking the generator when the batteries need charging you could just turn the power back on.