Winger Ken Sio scored four tries while Krisnan Inu kicked seven goals from as many attempts, with the other tries coming from Chris Atkin, Harvey Livett and Joe Burgess.
Marshall said: “Hull have ambitions of being one of the top teams in the competition, so to win in the way we did was very pleasing.
“I am ecstatic because we’ve had a tough season and to dominate a team like Hull was testament to the players.
“Attacking-wise, it was our best performance of the season. To come from that performance at Leigh (when Salford lost) to this in just over a week is some effort.
“This group of players have got some character and I am a proud coach. It’s been a tough week and to score that many points against that class opposition was brilliant.”
Marshall singled out his four-try winger for special praise, adding: “Ken Sio was outstanding to take his opportunities and score four tries against such good opposition.
“I thought the attack clicked and that process started against Catalans last Thursday. We were well organised in our attack and our discipline was a lot better.”
Hull coach Brett Hodgson said: “What really disappointed me was that we capitulated in the last 20. We are all hurting at the moment. It was really disappointing.
“It’s definitely back to the drawing board but we have got to find a way through that lack of confidence. We are missing some players and the confidence is diminished because of that.”
Asked why he thought his side had struggled in the final quarter, Hodgson replied: “Our set endings were very poor and we failed to apply pressure.
“It’s difficult to point out what exactly went wrong; we lacked resilience, which is something we’ve not lacked all season.”
When questioned about Hull’s chances of making the play-offs, Hodgson said: “We are too far from where we need to be.
“While there’s a mathematical chance we have to make sure we win every game. If we play like we played today we are not going to be there.
“We need to focus on the simple things. Rugby league is an easy game if you run hard and make your tackles.
“We just weren’t willing to do what was required. I thought we came here with a survival mentality rather than going after the game.
“I’m not happy leaking as many points as we did. To leak 42 points is absolutely not acceptable.
“Everyone will be hurting, fans included. Whenever there’s a chance we will keep fighting. There’s a possibility we can get there.”