You might be able to get a bleeper or buzzer wired in. Tow bar installers fit them when putting in trailer electrical sockets, they bleep to confirm the trailer lights are functioning correctly.
depending on wether factory fit or aftermarket some parking sensors have adjustable volume...(located on the main unit unfortunately this is often buried somewhere behind a trim panel)
I'd have thought that some mechanic with a bit of elecrical savvy, could pass the beeper wires on to an amp and then on to more powerful speakers. But you'd need someone with the skills. Or maybe they could pass the speaker on to a light display above the RV mirror (maybe) to tell you how frenzied the beeps would be.