Overall, we are very happy with our TankTender. In ten years of intermittent use, the unit only required one service call, and turnaround price and time were reasonable.
The simplicity of the unit and the good materials it is made from contribute to its longevity. The only tedious things to consider in the installation is the construction of a calibration table, and not to kink the semi-rigid tubing used to measure the depth of the liquid in the tank.
Since most integral water tanks are not shaped like rectangles, a "inches of depth" reading only becomes meaningful when you measure actual water in tank vs. the depth reading and affix the table nearby. Ditto for the waste and diesel tanks.
Putting kinks into the semi-rigid plastic tubing is pretty easy. Leaks or stoppages can result, either of which will make readings suspect. So think hard about how to route the tubing (that is unless TankTender has found a new and better way to do it).
The simplicity of the unit and the lack of energy required to run it make it very attractive for me. However, be sure to brief people on how to use it, as over-pressurizing the lines can lead to damage. I''d buy it again.