You don't need "pressure" to clean out a holding tank?? It's the volume of water that really matters when you actually dump it..
The main thing to know is to leave your black tank valve CLOSED until it's full..
Don't just hook up the hose and open the valves the first day you get to your site... I don't even hook up the hose until I need to dump.. And that's always on the day we leave 3-4 days after we get there.
I see people hook up the sewer hose, open both valves and think they are at home... ha, ha... Good luck with that after a few times..
Anyway, if you are learning how to camp from "youtube" good luck with that and hope it works out for ya!
Mitch