A farm and feed store may be able to provide "Bar Bait" (brand name) intended for Norwegian Rats. Very dependable and it causes dehydration of the dear departed which MAY (or may not) prevent the dead rat odor.
Some help available from Amazon:
In Florida we get rats and squirrels trying to set up nests in the trailers year round. Several of us have found that Pest-A-Cater brand electronic repellers seem to work. Cheap enough; plug it in to 120V shore power (in the basement of a fiver) and forget it; can't hurt.
If you do get a dead animal somewhere you cannot find, "Rat-Sorb" is the best we have ever found for removing the smell from confined spaces.
Good Luck!