I have the same problem from time to time... There are two possible issues that I can see (Really more but the rest are 2b, 2c, 2d, et-al)
1: Battery dead.. There are a couple of battery failure modes. one is "High resistance" and when this happens a new battery is the answer. (The other is shorted cell)
2: And was suspects go this is #1,2,3,4, and up to #(n-1) (The battery is #n)
Bad connection... Start by cleaning the battery terminals and the lugs that connect to them.. Even if it is a side terminal (GM type) battery.... I have had to do that 3 times so far on my coach. Battery still passes a load test with flying color (GREEN)
The rest of the "N-1" list is every other connection between battery and starter, both ground and positive side
Home is where I park it!