Whether slater is great because he has played in great teams, or those teams are great because of slater is a bit of a chicken and egg situation. It's impossible to answer.
To put a bit of perspective on it, by the time he made his australian debut, his storm colleagues Smith, inglis, Cronk, king, folau, hoffman, johnson, croker and kafausi had already played for australia. By the time he had made the maroons fullback spot his in 09 queensland had already won 3 origins in a row.
I think the sign of a great players is when they retire it takes ages to replace them. Sterlo retired over 20 years ago and eels have never had anyone fill his shoes. Joey at the knights, broncos have never been the same after Lockyer retired, roosters hit a few lull years after freddy retired. Storm have already found a replacement before slater retired. Have the storm really skipped a beat in his absence. Arguably with Munster at the back storm have been the strongest they've ever been over the last 18 months.
As for 'easily the best modern fullback' i think even that is debatable. Between 06-08 stewart gave him a real good run, you could argue stewart was better in that period. Karmicheal Hunt also had the Queensland fullback spot sown up and was a roo regular. 09-12 was definitely slaters best period. But even then hayne and barba won dally m medals in that time. Often wondered what would have happened if hayne and Slater swapped teams. Hayne and slater only faced off in a full origin series once, and hayne won ( almost single handedly at that).
In 13 and 14 there was a slight decline, errors creeped into his game especially under the high ball. There was talk about inglis replacing him at fullback in rep games at that time. Inglis run of form between 12-14 is as good as anyone's in the history off the game. In 15-16 he has become the forgetten man, as storm, qld and australia continue to dominate.
I think he is one of the great fullbacks, but the whole 'definitely the greatest of all time' is really debatable imo