Ok. So, standing on the shoulders of those who came before me, I believe I have come up with the DEFINITIVE way to successfully tackle this job, relatively easily and for under $3.00 to boot.

I started off by buying the Dorman 42055 PCV Valve Grommet that PNW Vanwagon found. After pulling the tank, cleaning up all the busted rubber, glue, and RTV the previous owner slapped on there, it became readily apparent that this isn't going back together using shear muscles or brute force. The float is physically larger than the inside diameter of the grommet, and the float, while not fragile in and of itself, slides on a plastic shaft which is absolutely fragile. Going all Rambo on it is just going to snap the sensor and that's it.

After staring at it for a while, and pointlessly trying to force a round peg into a noticeably smaller round hole, I had an idea. The float on the end of the sensor is the same size as an 18mm socket. So, first I muscled the grommet onto a 17mm socket as it was a little more manageable and allowed the grommet to stretch a little bit. Then I slid the grommet from the 17mm to an 18mm socket as pictured.

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I then just made the socket flush with the float and slid the grommet from the socket onto the float, using dish soap to help move things along. It wasn't quite as easy as it sounds, but it all came together in under a minute when it was all said and done. In-between all the pressing, cursing, and mashing trying to get this thing on, make sure you don't put the grommet on upside down, that would be really stupid. Once you've successfully completed this step, you're left with something that should look like this.

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Please note, this will not fit into the tank either as-is, the grommet will now be too large. No biggie, just slide the grommet back up until it's on the shaft right behind the float like so.

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Then feed the parts of the sensor back into the tank, one at a time. First the float, then the grommet, and then finally slide the rest of the sensor into place. The end product should look like this.

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While I haven't field tested it yet, it's apparent that I got a good seal on it. I'm letting it sit overnight in my sink to see if there's any moisture or leaks but, barring the unforeseen, it'll go back in the van tomorrow with a clean bill of health.

Hope it helps!