Installed in 2002, this unit was been drained and sitting the last two years. I have filled and tried to light the standing LPG pilot, but it won't stay lit upon release of the valve know push.

How long are you holding the button down? After you get the pilot lit, continue to hold down for at least 30 seconds to allow the thermocouple to heat up.
If that doesn't work you will want to try cleaning in and around the combustion chamber with a shop vac. Is your heater a FVIR model(sealed combustion chamber)? If so, I would recommend you call a Licensed Plumber for this. He or she should remove the access plate from the front of the combustion chamber, unhook the burner assembly and remove from the heater. Use a shop vac to vacuum out the chamber and then use a compressor (or some shop vacs can also blow air) to blow air through the air intake in the combustion chamber back out to clear any debris from the air intakes. Vacuum around the heater and then reinstall all parts and try lighting again. FVIR models need all the air they can get and a build up of debris can limit them enough to not allow the burner to function.