Furnace RPM's

[whutfles]

We have a 2010 Montana Mountaineer 326RLT. It is in storage plugged into 110V to keep batteries charged. I went out the other day to put new silicone caulking on the shower door. Temps were below freezing so I turned on the propane and turned the furnace on. Then I carried some tools into the RV and noticed hot air was comming from the furnace exhaust outside near the door so I knew the furnace was working. I had the ceiling lights on in the kitchen, LR and hallway. Then I noticed the furnace RPM's were slowing about every 10-15 seconds to about 3/4 speed for a couple of seconds and then return to full RPM's. When the furnace slowed, the lights were slightly dimming also. The longer I watched it, the slower the furnace slowed and the dimmer the lights dimmed. So I turned the lights off and the furnace continued to cycle fast to slow to fast. I was going to leave the furnace on overnight to allow the caulk to dry. But I was concerned I might damage the furnace motor if it continued to slow. The battery test in the panel said the batteries were fully charged. I checked the battery connections and there was no corrosion. I have 2 Interstate AGM 31M batteries. So after about 20 minutes of watching this, I turned the lights back on and the furnace started running normal and the lights quit dimming.

I read somewhere when I first bought these batteries that they can go to sleep and someone used a welder or something to put a heavy load on these AGM batteries to wake them up as they supposedly go to sleep. Is that what I have here? Did my batteries go to sleep? Or do I have some other problem?

We have not had the luxury of being able to plug into 110V in the past. Only recently moved to a different storage site. So in the past, charging the batteries was limited to plugging in the truck and running it for a while.