Negative air pressure in the trailer is pulling sewer gas up thru the toilet.
If you have a thetford toilet this is normal. They are NOT air tight by any means.
I actually talked them into sending me a new replacement free because I thought mine
was defective. The replacement is just the same as the original.
Any time we have negative pressure in the trailer we smell the toilet.
Example, cold weather, trailer shut tight, run the stove vent while
cooking breakfast. The smell is there in the bathroom due to air being pulled
out of the stove vent and replacement air finding it's way down the roof vent
stack and up thru the toilet.
Cure is to open a window somewhere while using the exhaust fan.
Donno why you get negative pressure when on the road.
Possibly you are getting positive pressure down thru the roof vent and that's blowing
the smell up thru the toilet.
There are vent toppers that have a venturi that sucks out sewer smell any time there
is a breeze. I assume these would also suck out the smell when you're going 60 MPH!!
Here's a link to one brand. They aren't cheap but folks say they work great.
Xtreme Plumbing Vents for RVs - Venturi Action - PPL Motor Homes