Cleaning a roof vent?

[dustyroads]

I own a 1959 Caravanner that has no grey or black water holding tanks. She has a straight pipe underneath that has a cut off at the end...not a whole lot of holding room. Soooo, when I use to run any of the sinks, water would back up through the shower...fixed that with a HEPVO and total new plumbing of the shower...now when I run the sinks or shower she drains extremely slooowww. I'm going to say a gallon of water takes about 30 minutes to completely drain out of the basin. My best guess is the one roof vent over top the shower. I have tried to see any obstructions to no avail...I have blown it out with a shop vac...put water down it...cussed it...nothing helps. HOW DO I CLEAN IT???? Is this my problem? Thanks for any advice out there!!!

[A W Warn]

I doubt it's the vent making it drain that slowly. I think there is an obstruction in the drain line.