I have a few of the details wrong (the power comes from radioisotopes, not fusion) but this is "Chicken Little and the Acme Little Giant" (1988) by Shirley Weinland, published in Analog, April 1988.
The titular Acme Little Giant is a compact power source:
Still, corporate management was going to need more than a set of maps before they'd believe in a sphere the size of a soccer ball that would run a whole house for three years—or, apparently, a tractor. After all, nobody advertised the Acme Little Giant, you couldn't order it from any TV catalog, and you couldn't look up the patents or read the scientific papers about it because there weren't any.
The story is set mostly at or around a truck stop in the southwestern U.S.
The story starts in unnamed city in California before the protagonist drives her R.V. to the truck stop.
I turned off 395 at the old shortcut and pulled into what used to be the parking lot for the Indian Wells Cafe and was now the entrance to the Indian Wells Cafe, Motel, Truck Stop, Airport, and RV Park—Angie's empire.
The protagonist is travelling in an R.V. and stops there; they may be secretly investigating it.
Not so secretly; she's pretty open about it.
I asked her about the Acme Little Giant. She looked at me sharply, then relaxed.
"Of course, you're retired—you don't work for Northern California Power anymore. It'll take two to move that beached whale of yours, though. I'll set up the appointment with the garage for tomorrow night."
"Hold on, Angie—I am working for them. They called me back as a consultant, to find out about the Acme Little Giant."
It is revealed that the truck stop is performing illegal fusion conversions for trucks (HGVs, not delivery vans).
It's not actually illegal (yet) because nobody officially knows anything about it. But lobbying might make it illegal.
I watched until both men had crawled out of the van, then I called in the accident on my CB—anonymously—pulled off the second transponder and set it to pickle with the first, and made my way sedately back to the truck stop garage, to get converted before anybody did make it illegal.
At one point the R.V. is travelling through a heavily-used tunnel and instead of intense noise and bad air the protagonist finds it's almost quiet because virtually all the trucks are converted, and have turned off their speakers and smoke-makers while in the tunnel.
The moment we were in the tunnel, the rig on my left was gliding along with no engine noise, only the sound of tires on pavement. When I rolled down my window to make sure, it was so quiet I heard Johnny Cash walk the line on their golden-oldies disk player.
I was the noisiest thing I heard. No wonder the "Dangerous Fumes Level" sign didn't light up. Out of the ten vehicles I could make out around me, there couldn't have been more than two others with internal combustion engines—diesel, gas, or synfuel.
At the end of the story I believe the restriction is lifted, and the truckers all turn off their camouflage.
Drivers who'd been in the middle of breakfast were scrambling to get outside, where truck horns were already sounding. Angie and I went out to watch as new trucks pulled in and drivers climbed up and ripped down their exhaust stacks. One driver hitched his sound effects speaker to his disk player and drove up and down the access road using his tractor as a giant boom box. People came out of their motel rooms to watch.
