Here is a cheap fix that can do wonders for squeaky Winny entrance doors. I found it on another site, tried it on mine, it worked, tried it on my buddy's, and it worked.
Take a look at where the latch wraps around the strike bolt when the door is closed. You might see some shiny metal area on the top of the bolt, and close to the mushroom head on the end of the bolt. If so, this is where the latch is rubbing, and squeaking, on the bolt as the rig rattled down the road. The cure is to take a bit of electrical tape and wrap three, or so, tight wraps around the bolt. In my case it stopped the horrible squeak, and it lasts for a while. When the noise returns I know that the latch has chewed though the tape and I need to replace it.