Re: new thetford portable holding tanks
Those blue tanks can be pretty effective, if used properly and carefully. There are two different approaches that work well. The first is as mentioned, to only get a size that will hold at least a little more than your larger holding tank. By doing that you can always be sure to be able to fully drain the holding tank into the tote tank. The problem with that is the size of tote tank that you will need. Remember that the waste is mostly water and so will weigh about 8# per gallon or a 40 gallon tank when full will weigh in at about 320#.
Another approach is to use a smaller tank and empty it more often. That is what I do when I use one since I don't want to fool with 300#+ and I have fourty gallon holding tanks. I got one from Thetford and I also got the float level indicator that serews into the vent/hose connection on the end of the tank away from the large sewer hose connection. I then just keep it connected to the gray water tank with the dump valve open. Once that tank level reaches about 3/4 full, I close the dump valve and take the tank to the dump station, I then empty the black tank as well since it does not fill as rapidly as the gray tank and that means that I also have the space to flush the black tank into the blue tote tank as well. Of course, it is still important to watch that level indicator.
I use a twenty gallon tank which serves us well and it fits into the storage area that we have available. By doing that, it means that the most my tank will ever weigh is about 160#, which is as much as I care to fool with.