Well, I managed to do a "smoke" test today, using an incense stick, as suggested by jheacox1, and with an assistant.
Unfortunately the black tank vent is the one right behind the bathroom fan vent, as suspected, so it looks like I won't be able to install one of those cyclone vent pipe covers.
Haven't had any real issue, but was just considering it as a preventative for when in extreme high temps during summer trip
2015 Rockwood Signature UltraLite 8281WS
2016 Ford F-250 Lariat CrewCab, 4x4, 6.2L V8