If this is an ADA rated bar, unless the screws go into blocking or studs, just the expansion anchors will NOT provide the strength required to meet the requirements. A neat anchor that WILL provide the required strength if you can't hit blocking or studs is a device called a Wing-Its. They're not cheap, and you have to be pretty accurate in drilling the required holes (one on each end), but it is guaranteed not to leak and to meet the ADA safety bar requirements.
http://www.wingits.com/Grab-Bar-Fasteners While they are designed to work with their bars, the hole pattern is pretty standard, and I've had success with a different brand (right now, I can't remember the brand, but it was definately not one of theirs).
What I found worked best for me was to drill a through-hole in a piece of ply, and position that on the wall, then used that hole to drill into the tile. It might work out best to actually drill both holes in the template, ensure the bar will fit properly, then drill the wall. THere's a rubber gasket with sticky adhesive on the back side of the flange and rubber feet on the spring-loaded fingers that spread out after you insert it into the wall, and a collar that exactly fits the hole. It's all SS. Once it's in, it's in, and not coming out without tearing out the wall, so if hole alignment is important, make sure it's right! It's really easier than it sounds, but you do have to be exact. The fingers on the back spread out to maybe 3", which along with the flange on the shower side, makes for a very secure anchor. It easily meets the ADA requirements with cbu and a layer of tile.