Dear Readers, in response to a query in an earlier column, I received a couple of emails regarding the use of a vacuum truck operating at a low vacuum to evacuate the holding tanks on the RV. There was caution alerted to the fact that if too much vacuum is exerted, that which exceeds the flow of air into the tank via the tank vents, the tank could then collapse. This is true. However, it was assumed the operator of the truck, (the writer of the original question), would understand the emphasis of using a very low vacuum for the evacuation.All that said, the old Doc would like to add this addendum to the original request for the correct procedures. In addition to verifying the tank vents are indeed clear of obstructions, I would also recommend that the toilet flushing mechanism be held open to aid in the venting process of the solid waste holding tank. In the past I have an ABS tee fitting to keep the toilet flush valve open and still allow air to enter the tank at the same time. Also be aware that it will more than likely be necessary to add water to each of the “P” traps in the gray waste system after evacuation. Many thanks to those alert readers who voiced their concern.
(Note: To read the original question, go here.)