MarkR,
Both my '06 and '87 have only one vent pipe protruding from the roof. And both of my trailers have anti-siphon devices in the drain pluming. So you may be able to get by with one vent pipe as long as you use one of these anti-siphon devices in your drain runs.
I got this pictures
From this web site
Vent Pro - RV Parts, RV Supplies & RV Accessories for 5th Wheels, ToyHaulers, Campers
But I will post better pictures of one of the anti-siphon devices that is installed in one of my trailers when I get home from work tonight.
Hopefully you didn't cut that second hole in the roof yet
