Ramp door leaking

[1231ChaseB]

Hi All ! Yesterday i got tired of looking at the rusty washers on the ramp door hinge's ! Both inside and outside ! Anyway as i changed these things i was amazed at the amount of rust on the bolts themselves ! Obviously there has been water inside of the door itself ! Not Good ! Has anyone had anything like this and how in the world could water get inside of the ramp door ?It's looking like i need to do some sealing but i don't know where to start !
Thanks in advance
chaseb