I've got this problem on a Cat 613 that I'm driving at the moment, someone had left the diesel cap off as well and the birds had nested in the top tank which is part of the chassis, I used a wet vacuum cleaner with different shaped hoses as well as a piece of wire to hook and suck as much as I could out plus removed the bottom tank and washed, sucked and hooked all the crap out of it, what a nightmare!
Good luck.