$135 for an inspection port retrofit kit???
Ronco Plastics has 4" and 6" inspection ports for $25 and $30! And their price includes installation in a new tank ordered from them, but they're very easy to retrofit in an existing tank yourself. http://www.ronco-plastics.com/Beckson makes 'em, but they don't seem to be on their website. However, if you contact Beckson, you might even be able to find 'em for less than Ronco's price.However, I agree with Rick...you don't really need one unless you regularly have guests aboard who insist on disregarding instructions as to the only things that can be flushed (what you've eaten first and limited amounts of quick-dissolve TP). Flushing out a holding tank can easily be done by putting a few gallons of water at a time down the pumpout fitting to stir up any sludge, then repeating till what's coming out is clear water.