NiceAft wrote:What I am querying, is attaching the hoses to the threaded protrusions of pipe when the caps are not used.
The big nut uses a hose adapter flange, sort of like the a drain fitting for the bottom of a sink basin. The one you have is made by Mueller (I recognize the logo), and it has a 1-1/2" National Pipe Straight Mechanical (NPSM) thread. Probably made for a tailpiece connection on a sink drain.
West Marine sells a 1-1/2" pipe to sanitation hose adapter, that looks like it has a straight mechanical pipe thread (but I can't swear to it), that might work to directly connect to that male thread. WM #285876. If it's a straight thread, it may need a flat washer to make the seal. Use soft rubber, not fibrous.
I don't know what size the vent is, but you could probably find something similar.
The nice thing about using the nut with barb spigot is that you can leave the hoses behind (with the nuts captive on the fittings) and take the PP out for the winter, and clean it. Not so easy with a hose to female mech. pipe connection, but if you lube it, you can probably loosen the hose clamps and spin the connection to get it off the holding tank.