That Garmin has "true RS-232 output" levels
whatever that means - RS-232 space (0) is +3V to +15V, mark (1) is -3V to -15V
RS-232 on proper cables is good for 75 feet at 9600 baud, and empirically more.
Voltage drop on good quality cables over 5m shouldn't be too bad. Might be worth measuring what actual signalling voltages that Garmin actually uses, then run the numbers, check the signal is still > 3V and < -3V (i.e. within RS-232 spec) by the time it has traversed your cables.
If you need to get commands up to the GPS, you might struggle if your UART only signals at 3.3V. You might well drop more than 0.3V over 5m, and the Garmin might not tolerate running out of spec.
Good quality cables with twisted pairs and proper shielding should stop any RFI problems.
Might just be easier to buy it and try...