The starting ok when warm phenomenon could indicate either malfunctioning glowplugs or low compression. If you have new glowplugs and 12 volts at the plugs when the switch is in the glow position, it points towards low compression. It also means the relay is working. You could keep the car running 24 hrs during the road trip.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13