Thank you all for your responses.
From what I've observed in your responses; I think that if I ever wind up living in a trailer home, I'd replace these hard-to-get things with old-work plastic boxes with the "swing out" tabs that fasten as you tighten the screws.
That way I can use standard receptacles and switches available at any hardware store.
I don't know if these things are commonly available in other parts of the country, but I've never seen them sold here in New York City....well considering we don't have trailer homes here and all the construction site trailers and temp-office school and office buildings all use standard sockets.