If you have an enclosed underbelly it's possible they were in there. Besides the smell from decaying carcasses, there could be pee & poop in the batt insulation. I would remove the underbelly (if so equipped) and replace as needed. Odor could be coming up through any holes through the floor for piping, ducts, etc.
Mice have poor eyesight and navigate by constantly dribbling a trail of pee. So if they're inside or under an RV, there can be pee all over, not just an isolated spot or area.
I had to pull part of our underbelly coroplast off last year. Found one very shriveled up carcass. Never smelled anything. Mice can squeeze through a hole the size of a dime and get into spaces you wouldn't expect. A few years ago I sealed up ALL the openings & cracks through our floor to keep drafts out in cold weather. Good thing because the mouse and his friends might have gotten inside.
Here is a little info. on how to get rid of mouse pee smell in RVs. Apparently a black light will show where there's been pee.
