an old truck pal of mine from New Hampshire has this suggestion:
What worked for us up north was to shoot a bunch of graphite into the lock itself (buy cheap at hardware store) and use lithium grease on the hinges, clasp and inside the door panel on the locking/door handle mechanisms.
Do NOT use WD40, PB Lube or hot water!!
No idea why, but the spray lubes always make it worse.
