Is the vent just a simple vent or does it have a powered fan, too? Hopefully it has a fan so that you can turn it on a push air out rather than potentially having holding tank gasses come in. If you have the vent cover oriented correctly in your picture, adding the vent cover will eliminate the potential problem if you don't have a fan in the vent.
Even if you extend the vent pipe you won't likely be able to use one of the hinged vent covers.
If you contact FR about the design, let us know what they said.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)