The problem with new woodwork in an old trailer is that even if you had the exact same stain used 40 years ago, it wouldn't match. With age it oxidizes and changes color based on the enviroment.
The best bet is as Davydd suggested, take door panel or drawer in to have it matched. Be prepared to possibly have to use 2 or 3 different stains over one another to get the desired finish.
Be prepared, in a couple of years even if they "match" today, the new panels will age and change differently than the 40 year old panels...just something that can't be avoided unless you refinish all the woodwork alike.