It's ok to be a powerboater here...I'm one!
I've never known of a PE tank to permeate. 90% of the time, when somone has been 100% certain his tank has permeated ("I cleaned it out, brought it home and stored in my garage and the whole garage stinks!"), when asked, "Did you plug the fittings?" the answer has been no. But you've sealed your fittings...so that leaves odors attached to the outside of the tank. There is a product that will cure that: PureAyre
PureAyre You can find it at pet stores, Amazon and direct from the company. Any variety will work. I’d also buy a good quality 1 gallon garden pump sprayer jug (you COULD use a trigger sprayer, but your hand will get very tired).
You cannot eliminate any odor unless you first eliminate the source of that odor! Since you've already done that, use the sprayer to lightly coat every surface, nook and cranny in the affected area with the PureAyre.
DO NOT RINSE! Just let it dry, leaving all hatches open--even turn a fan on if you can--for at least 24 hours. If you still have any odor, you missed a spot.
Also use PureAyre to eliminate residual odors left in lockers by permeated hoses after you’ve replaced them. If cushions or other soft goods were affected, clean them and then saturate (not dripping wet…just enough sprayed into the foam from both sides to penetrate to the middle of the cushion) with PureAyre., then leave outside in the sunshine to dry all day.
And by the way, PureAyre is safe for use around children and pets...and food. So it’s also the best way to get rid of residual odors (the meat that spoiled when the power to the fridge died, the sandwiches you forgot to take out of the cooler) in bait boxes refrigerators and iceboxes, musty wet lockers and anywhere else that stinks. It’s also the only product I’ve found that eliminates diesel odors—after you’ve cleaned up that spill!
I first found PureAyre at a boat show in Seattle in 2005...talked the mfrs, came home with samples. I've been a fan ever since. When my dad was in a nursing home, I did his laundry...I bought PureAyre by the case! It's the only product I've ever found that will eliminate ANY odor, even diesel and smoke.
Btw, you might want to check out the link in my signature here...includes a lot of informration you might find useful.