: I am installing a new toilet in a Park Model Travel Trailer that was manufactured with out the standard RV unit.
: I am looking at the Briggs Vacuity toilet

: Question: is it better to plumb in 4" piping and a 4" toilet flange or 3". Any benefits of one or the other sizes? Most RV plumbing is 3"

I think you pretty much anwered your question.
Use the 4x3 flange with 3" pipe.