Road trip movie from the 80s?

There's this movie I think from the 80's (maybe a little later) where these two guys are given the task of transporting a politicians "crazy"" daughter to a mental facility.

What I remember from the movie is the girl escaping, telling a truck driver these men are trying to rape her. Then later the guy says he'll take her to her mother. The mother rejects her at first but then gets her out of the hospital. I remember the mother had a P.O. box and in order to get her real address he tries to send a packages priority over night mail to the same post office he's sending it from which for some reason is a big hassle to the post master and he gives them the address. And I remember the mother threatening the father saying she'll have his ___ in the pencil sharpener if he gives her trouble.

I remember almost the whole movie but not the name. Help!