Anybody that thinks Murray wins a load of slams in any other era (never mind 12) is just either dismissing his first 2 slam finals until Wimbledon 2012 or didn't see them. It was so clear that he had bad stage fright until he played well in a loss at Wimbledon 2012. Not only that, he would never win double digits in any other era because his second serve and FH were average at best. His FH (and mental) improvement under Lendl is the main reason Murray has the slams he does, and I'm very happy for him. I've always liked to see him succeed, but he was never winning double digit slams no matter when he played. That was obvious from the start to anyone who paid attention. He might've had a Becker/Edberg slam total, and that's being nice IMO.
Impossible to compare across eras, first of all, but there were many good/great players in the 80's-90's to stop Murray from winning slams. Starting with Sampras and prime Agassi in the 90's for two, lol. Also, no surface homogenization So I'll focus on 04-07.
Roddick beat him at Wimbledon in 2009 when he wasn't even in his prime, really. He just had one last great tournament and that was it. Not even mentioning guys like Hewitt, Ancic, Scud, Grosjean, Johansson and his compatriot Henman. So even without Fed (and also Nadal to be nice), it's far from a guarantee that Murray wins Wimbledon in his early years. He probably has to take a loss or two in a final to Hewitt and/or Roddick before he actually wins Wimbledon so it's not that much different from his true career trajectory.
His USO performances are up and down to say the least. He won in 2012 sure, but he has a lot of early losses for a player of his caliber as well. And even without Federer and Nadal he has to beat a very good, if old, Agassi, Hewitt (a former champion), Nalbandian, Davydenko, Roddick again, James Blake, who beat Nadal in 05. Even Ferrer who beat Nadal in 2007 was never an easy out for Murray. Remember, Murray has losses to Cilic, Wawrinka (in 2010 and 2013 so pre Stanimal, especially 2010) Anderson, and Nishikori on his record at the USO. Pretty good players sure, but not at all noticeably better than the aforementioned group, worse tbh.
He might win an AO, maybe 2 considering he was really good there, but even there he'd have to beat the same guys mentioned above that he would probably struggle with at the USO, and that too on Rebound Ace, not plexi. On plexi he'd probably beat those guys, but on Rebound Ace it's far from a guarantee. And I shouldn't forget Safin who was awesome at the AO, and guys like Baghdatis, Gonzalez and Tsonga who had the tournaments of their lives there. Lots of depth of competition in 04-07, just not top heavy like it's been post that time.
And re Murray and RG: Not even worth discussing.