As far as useful information for this problem I would question a little bit if there is a problem getting rid of heat up front since running it down the road is improving things Maybe the ac condenser and or the rest of the stuff up there is restricted on air flow. Not sure why that would make the bunk still work good but it’s a thought
That Shoreline box is a quality piece and made in the US. Mine was not an exact fit on the 94 W9 I had but all I had to do was egg the holes out a little to fit. Start by making sure you have a full charge. Should be around 4.5lbs or so. Turn off all coolant valves leading to either heater cores and make sure your condenser is clean. Manually turn your fan on to. If it's not cycling that could be an issue as well.
