Yeah, that line is completely exposed to the elements under the slideout and it is a price you pay for having that residential refrigerator.
I suppose you could try and insulate with some pipe insulation when the slide is out but you probably would have to remove it before retracting the slide.
The other option is to winterize the ice maker/water dispenser and go without when cold camping by blowing out the lines for the water dispenser, then close the supply valves. On our 330RSTS, there are two. One under the island that stops water before it goes thru that exposed line, and one behind the drawer under the stove that is just before the water enters the refrigerator.
I am sure they would be somewhere else on the 338RETS. If I had to guess based on where ours are, the supply line valve is probably under the sink in the corner teed off the cold water line and the other probably the same place our is, behind the drawer under the oven just before going behind the refrigerator.
2018 GMC Sierra Denali 4WD LB SRW 6.6L Duramax
2017 Jayco Eagle 330RSTS