Displays with 4.3" and 16:9 ratio like the one below are available from Chinese outlets for less than $10. Resolution is 480x272 but it's not clear if that's the raw resolution or actual RGB pixels. Power supply is 12-24V (actually 9-34V). Two video inputs are provided, one has priority. If used with a rear-view camera the lower-priority input can be used as a status display. If no video input is provided the display is in standby.
Assuming that the native resolution of the monitor is 480 lines tvservice -c "NTSC 4:3 P"
should provide a stable image for direct TV-out settings. The commad also shows the corresponing sdtv_mode
value.