One of my biggest questions is if there was normally a ladder on the outside of the tanks. Also if there is a ladder on the inside the tanks which would have made access easy or not. It was reported earlier that these were metal tanks. The manufacture of the tank might of installed or welded rungs on the outside and inside of the tank to be used as a way to enter and exit the tank for maintenance purposes or maybe not. All the pictures I have seen don't suggest that there are permanent ladders installed on the outside of the tanks.
If Elisa did manage to climb up the tank and fell in or climbed in, she might not have been able to get back out if no ladder existed inside of the tank. She no doubt would have drown after losing strength to keep herself a float.
If she was placed inside the tank by someone while she was alive I think bruises and abrasions would be noticed during postmortem examination. Looking at the configuration of the four tanks and the height it would be an incredible feat for someone to carry a body up there and place the body into the tank.
I have examined many photos on the web and cannot determine if the ladder seen leaning against the tank where Elisa was found, was placed there by the firefighters or if this ladder might have been placed there by the maintenance man or been there all along. Without this ladder I don't see an easy way for an individual to climb the tanks, much less for someone to carry a body up to the top of the tank. This is no doubt an aluminum ladder which makes me think that this is NOT the fire departments ladder since aluminum ladders have a higher risk of electrocution. I just can't see the LAFD still using aluminum ladders. If this ladder was already present on top of the building the fire department might of opted to use this ladder since to get their own ladder to the top of the building, the ladder would have likely had to have been carried up the stairs or hoisted it up the outside of the building using a rope, since a ladder of this size I don't think would have fit inside the elevator. I would imagine that this ladder was used by the maintenance man who originally discovered Elisa's body in the tank.
If the only way to get on top of these tanks was by use of this ladder and if the ladder was normally stored inside what looks like a small building on the top of the roof and placed there by the maintenance man when he went to examine the tanks in regarded to the water pressure problem, than someone with good knowledge of the hotel must of placed Elisa in the tank since Elisa couldn't have moved the ladder after being inside the tank. Who would have known were the ladder was and who would have put the ladder back where it belonged?
I have read in the comments left by past guests of the hotel that water pressure was an issue and perhaps this ladder was left standing against the tanks on a regular basis. Regardless I think if that ladder was kept there on a permanent basis is a big clue to this mystery.
JMO