I think Elvis' creativity was somewhat stifled when he returned to Vegas soon after the big event of "Aloha From Hawaii". It was very shortly after this momentous occasion that Elvis' weight fluctuated. Another contributing factor was that he knew he not only had another August/September season at the Hilton to get through, but he was also performing a short stint in Lake Tahoe once again before that.
I don't think Vegas was very good for Elvis' state of health or mind - I feel that playing the Hilton the number of times per year that he did caused him to get stuck in a rut. It was like a parallel rut to what he was caught up in during the sixties making those movies.
It's quite obvious that 1973 was a big turning point for Elvis both professionally and personally. At the start of the year Elvis became 'Mr. America' so to speak - he was invincible. But, come the end of the year he had to come to terms with the emotional upheaval that divorce brings. It was the realisation of the start of a new era.
I think that Elvis embarked upon a lot of creative soul-searching between December 1973 and December 1974, and to a certain extent he didn't really find it. At this point I feel the only time he was happiest was when he was touring and playing to bigger audiences in different arenas each night. But, when the fateful September/October 74 tour came around, this was certainly the point of no return.
Elvis' usual Vegas stint was cancelled in January 1975 - Elvis was in hospital. So, by the time the month of March appeared Elvis had been somewhat resting for a 5-month period, which is what he had needed. The creative light was switched back on that month with the recording of the 'Today' album in Hollywood and then a revitalised short Vegas season.
It is quite obvious though that things were never really the same again with Elvis after then - his health and medication dependency were going to combine to start dictating the rules.
Just my thoughts.