The Robins upset the odds by knocking Warrington Wolves out of the play-offs with Friday’s shock 19-0 win at Halliwell Jones Stadium.
They have to travel again to face the League Leaders Shield winners in Perpignan but know another victory will leave them in the Grand Final for the first time.
Second-row Johnson, 22, said: “We’ve said the past few times we’ve played Catalans we’ve only been beaten by a few points.
“We fancy our chances and we’re all buzzing about how things are going. We’ve spoken in the last few weeks how we had to split it into halves.
“Last week was eight halves. This now is four. We’re just buzzing and taking it week by week.”
Rovers were 30-18 ahead on their last visit to Perpignan in July but lost 32-30 and then they narrowly fell 23-16 at Hull College Craven Park just nine days later.
KR secured the sixth and final play-off spot in the final regular round and they finished bottom last season but are now contemplating a remarkable turnaround.
Their defensive resolve and team spirit against a Warrington side featuring the attacking quality of England trio George Williams, Gareth Widdop and Daryl Clark was immense.
Victory was particularly pleasing for Leeds-born Johnson who, unwanted by Wolves, left them for Rovers in December in a swap deal with prop Mulhern and has become a regular.
“I had a few of my family saying it’d be unreal to win there and it felt good doing it,” said Johnson.
“It’d be great to go on and do it (reach Old Trafford).
“On Friday, it was one of those we had to grind out and we knew straight away we had to get some points early.
“Once we did that, we then managed to hold them out and there were some brilliant efforts.
“The grit we had was excellent. Especially from everyone in the middle when it was getting tiring.
“We said all week Warrington were a dangerous side and they had loads of dangerous players so it was good to keep them out like we did. It bodes well for us.”
Meanwhile, Johnson’s back-row colleague Kane Linnett is the club’s sole represenatitve in the Super League Dream Team.