While mounted you CANNOT do this:

/target X
/focus

you CAN however do this:

/focus X

This is because while mounted you cannot "target" players by name, only their mounts. I use the standard blizzard UI, and just do two things:

If you setup the following macros and keybinds it makes everything easier.

Macro1 (i have mine bound to F1)
/focus tankname
/p i set focus to %f

Macro2 (i have mine bound to f, for follow!):
/follow focus

Macro3 (i have mine set to `) this macro changes the target on all your slaves to the tanks target:
/assist focus

finally in keybindings go find Interact with Target and set it to something (I use shift-`, or ~).

Voila -- press F1 to have everyone set their focus to your tank, F to follow, ` to assist the tank (change targets), and ~ to go run up to the target in case of fears/etc.