At least with the RV type fridges, they like to be reasonably level for the refrigerant to circulate correctly. They don't have a compressor, so as I understand the ammonia type refrigerant relies on heat to start the process and gravity to continue to the flow. Down the highway doesn't count, because at that point the motion itself promotes the circulation.
I actually have no clue if the residential refrigerators are sensitive to that.
Dave's here - he pops in at times.
Brad & Dory - Lone Tree, CO
CURRENT - 2013 Itasca Meridian 42E/2013 Wrangler 4dr Sahara
SOLD - 2014 Redwood 36RL/2014 F350 DRW