I have the shallow VTS grey water tanks and installed the short SeeLevel gauges on them. I was told by the SeeLevel tech support guys when I was laboring over the decision, that as my gauges will cover the part of the tank where the wall thickens up as it approaches the corner, there may be some accuracy issues. I ran a bench test and slowly filled the tank with water, and it looked like they were pretty darn accurate, certainly accurate enough for the application. Can't offer any long-term performance anecdotes, as I am still laboring away at completing the rennovation.
One thing to be aware of is that you have to place the gaues strategically so that they are not near any metal. Proximity to metal screws them up.