Between the point when the tank was manufactured and then later installed in your trailer, the tank was most likely stored outside...possibly for a very long time. My guess would be that the bugs entered during this time.
There is a place near me that manufactures tanks, and all their inventory is stored outside year round.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)