I think that sometimes when moving the gases can get sucked back into the rig...but here's something others may know that I only just learned recently:
Some RV toilets have an "overflow port" under the rim of the toilet. (In case the bowl fills up with water somehow.) Anyway, that hole has a "j-trap" that prevents sewer gases from rising back into the rig- but if there's no water in it, there's no seal.
Supposedly, if that trap "sloshes empty" when underway, you'll get the smell in the rig.
Might be a long shot, but maybe worth checking out...especially if the smell was stronger back by the john.
" Not every mind that wanders is lost. " With apologies to J.R.R. Tolkien