Check the hitch and pin, check the trailer plug and Anderson connection. Check the safety chains.
Then down one side checking each latch on boxes and camper, rear door, up the other side, then get the missus to check the lights while I operate them. I'm the driver, so I'm responsible, so it's my job to check. The 6 camper floor latches stay undone when in the garage to prevent the seals from becoming overly compressed, so they must be closed before departure.
If we'll be stopping along the way where the tug/camper would be parked and unattended, as I do the check, I have 12x keyed alike padlocks, and I fit 1 on each box, 2 on the camper lid, lock the back door (keyed lock), 1 on the pole box, 1 on the rear locker, and replace the hitch pin clamp with a padlock. Have the same locks on the 2 gas cylinders. 10 locks, 2 spares .... Keys stashed everywhere !!!