Welcome to iRV2.
Check the RV battery (ies) , connections , electrolite level , and even take it out for a load test. If the batteries and connections check out , then try to get a voltage read out, at the batteries during the dim period , you could be looking at the earliest signs of a converter/charger failure.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.