I'm assuming this is a bumper pull? Most 5ers have pass through storage. My old tow behind had the same problem. I added a cheap set of drawers from Wal Mart for our clothes just inside the door, that helped. Aside from that the back of your truck with a cap or tonneau cover is the next choice (I had a locking tonneau and it worked great.
But with a 1/2 ton truck be careful, won't take much to be over your GCWR.
DIRT BIKES RULE
'12 Duramax CC short bed
2019 Wildcat Maxx 285RKX