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When is the last time you had the batteries serviced and load tested? If they are over 3 years old there is a good chance that they aren't going to be good and hold a charge. Surface voltage doesn't tell you the actual condition. That can be just surface voltage. When you try to turn the engine over does it just click or nothing? Did you have any of the battery cables off recently? Get back to me with more information here and I can assist you further. All follow ups are included in our fee.
Even brand new batteries fail quite frequently...........just had one a few days ago where one shorted out internally and ruined all the rest and they were less than a year old in a brand new coach. Were any of the cables removed recently? Were they discharged and perhaps they froze? That ruins them as well. Do you get any click when you try to start it? Gauges come on? Tracking this down if the batteries are good will take a 12 VDC test light or small digital multimeter for some testing.
It could be any number of things and if you can get a small digital multimeter we can do some testing. I do think the batteries should be load tested to be sure they aren't the issue. Has this started since you replaced them? How long has this been sitting? Any chance mice or other critters may have gotten into this.