Wow....unfortunately, I've seen worse...just not lately. As DrewE said you probably swapped out the coach batteries.
Find the starter and follow the cables back to the battery set. Along the way, check for badly corroded or missing or shot fuses and connections.
As noted, about any backyard diesel mechanic should be able to get you going.
But you got a lot of work to do in that battery compartment. Clean as much as you can to expose as much as you can. If you can't see it, you won't be able to tell what's obvious and what's hidden.
Jim
2019 Coachmen Mirada Select 37LS