For RS232 level shifting to 'safe' empeg levels, I'd use a Dallas Semi (now Maxim) DS275 RS232 transceiver.

Its a small 8 pin device, which steals its TX line 'idle' voltage from the RX line from the device at the end of the RS232 link [i.e. the empeg], thus removing the need for voltage invertors - which the MAX23X series provides.

This makes for a small and simple RS232 level convertor, without the need for capacitors that some MAX23X chips need.

all it needs is +5Volts - already needed for the GPS.

Of course if you want to provide the RTCM input as well on the second serial port on the GPS unit, then you'll need a second DS275, which may make a MAX233 a better option as it has 5 transceiver circuits, enough for TX and RX for both serial ports on the GPS unit.

You could probably power one of these DS275s from the serial port pins on the EMPEG [the DTR line is +12V] with a 7805 low power regulator and a cap.

Heck you'd probably be able to power the GPS unit from the same circuit if the 7805 was properly heatsinked and the GPs board didn't need more than 400ma.

I'm also sure you could set things up so that the GPS was powered and only powered down when the Empeg itself went to "sleep" [i.e. led stops blinking], thus allowing you to keep your GPS "engine" running [and ready to reacquire its position much more quickly] than from a cold start like when the power has just been turned on again, useful when you make a short stop and don't want to lose your GPS "lock".