Over the years, have seen the large-to-small adapters around, would not know where to find one. Slip-joint PVC reducer might just be useable. Sure couldn't walk away from it at any time in use, tho.
On second thought, flow rate on the truck pump may be too high to use a reducer.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired