cheeze1 - I was using my imagination, but thought I could be wrong.
The type of backup alarm sensor you are looking for should not be confused with the backup alarm that starts beeping when you place the transmission in reverse. I did a search on eBay for proximity sensors and found close to 100 devices - majority under $120. Most are described to be used with burglary alarms. Several mentioned setting sensitivity, I would have to learn more about them. Could the RV be moving and detect a stationary object getting closer? Suspect that it could. Some mention a red light coming on before setting off an alarm.