+1.
I'll hate myself for saying this as it is so over used by some people, but it is one situation where you really do not want to resort to equipment to overcome a skills deficiency.
In my opinion, a slob knob should only be used in a situaion where a diver has a shoulder injury that precludes ever having the range of motion to reach one of the valves - and frankly I'd recommend they side mount before I recommended they use a slob knob. They add complication, reduce streamlining and will frankly result in other divers pointing at you and whispering even if the knob does not really sneak up on you and strangle you in your sleep.
Barring an injury, if your suit fits properly and is not overly squeezed and if your tanks are properly positioned in the bands and on the plate, you should be able to reach the valve knobs - or at least reach the first stage and then walk your fingers out to the knobs. The isolator is much easier to reach as it is on the centerline.