While cleaning my office yesterday, I found an old copy of The Humor of the Fabliaux, and was struck by how many of the stories had wandering clerks (sometimes known as episcopi vagantes) bedding down the wives and daughters of peasants.
This got me to thinking -- could the stories of episcopi vagantes and peasants' daughters be the ancestors of the modern "travelling salesman & farmer's daughter" jokes? They are certainly of the same type, but the types might just seem similar because they are types. After all, the naive rural girl (or the faux ingenue) is a stock character in a lot of stories, as is the travelling male predator, but something about the way the fabliaux so often poke fun at the relative social distance between the clerk and the peasant girl (or her father) seems very similar to the social distance between the worldly salesman and the farm girl (and her farmer father).
I'm not sure how one would even go about proving the link -- I'm reminded of Mike Barnacle asking how one sources a joke -- but it's certainly intriguing, and a reminder that medieval people found a lot of the same things funny that we do.