OP
Manfred you need to update your signature-appears many are responding to it and topic title and not reading post & question.
I'll further muddy the water,
I'm reading question is about powering refer between point A & B, on the road with 12v, starting with charged batteries.
Between solar and truck alternator, all it needs to do is supply at minimum what refer draws.
Biggest factor in using 12v on road IMO is never using without charge source (engine running). Turning on refer last, turning off first.
Its that draw down that system likely will have a hard time with-supply adequate charge while supplying ~16-19amps to refer.