I had a similar problem with ants. Bombed them twice last winter just after I got the boat and while I cleaned and worked on the neglect (it had spent two years in the previous owners backyard). Those buggers survived even after both bombings. I then moved to the borax powder and the fast kill sprays. I thought I had located the nest and was unmerciful, spraying every time I left the boat. Population went down but I still found a few every time I went onboard. They were my constant crew (and better behaved them some I’ve had).
I have no idea what they were eating. I had gutted the boat to recondition it. Even went so far as to pressure wash it inside after bleaching every surface (mildew everywhere). I had brought no food onto the boat. The bilges were clean as was the rest of the boat. I had removed all cushions, fabric, and carpet, and still they persisted. I launched my boat this spring with a full crew.
Finally the weather warmed up and the sun was out, enough that I opened the bimini (which I loath, but my wife can’t take as much sun as I can) and lo and behold, there was the nest, nestled in the folds of the cloth. I knew it was the nest because; 1. There was a very large ant in the center of the mass of smaller ones (queen), and 2. After I knocked them overboard my ant problem went away. They weren’t there when I launched the boat (the bimini was still in the garage) but they must have moved it there with my constant onslaught of insecticides inside the boat.
With all of the hidey-hole’s inside a boat, pests will be a constant battle. And not to rain on anyone’s parade, but cockroaches fly, as do ants. I wish you all the luck with toxic (to roaches) docklines, but the only way to keep them beat back is with constant vigilance and help from the chemical companies. “Better living through chemistry”.