I don't think there's a trend at all. Seems to be mix and match and more weight driven and routing related than anything I bet. Last camper was an HTT and all LP lines were soft copper with the exception of the pigtail for the low pressure BBQ. New camper, same manufacturer, higher and heavier line - 3/4" black pipe front to rear. Quick connect right off the pipe for the grill. HWH and furnace are fed from very short soft copper runs teed off the black pipe. Fridge in the slide-out is the only appliance with a true LP flex line.
