I guess I'll have to put in my 2 cents worth, since trailering is something I have lots of experience with.
1. A fishtailing trailer is
usually the result of too little tongue weight, although a flat, or very low, tire can cause it, as can a sudden cross wind.
2. Letting off the gas suddenly may make it worse, and suddenly hitting the brakes almost ensures a crash.
3. Accelerating
might help (I'd even say
probably help) for the immediate moment, but no way I'd do that without trailer brakes. Instead, ease up on the accelerator and gradually slow down.
4. And
if you have trailer brakes, then you definitely want to apply the trailer brakes while simultaneously accelerating very slightly and you'll pull everything back straight. I once drove over halfway across South Dakota in cross winds keeping one thumb on the trailer brake controller.
5. Any RVer who has pulled a conventional travel trailer has undoubtedly had 18-wheelers pass them, and when they do, it feels as if the back of the towing vehicle/front of the trailer is being sucked toward the 18-wheeler. It can really scare a person new to trailering, even to the point of them hitting the brakes, which is the worst thing they can do. In that scenario, nothing needs to be done at all. The rig will almost immediately straighten itself, although it does not hurt anything to accelerate a bit.