There appears to be something strange going on here. The grey tank on my 5.0TA is also listed at 28 gallons. At the end of my last trip (last summer), all tanks were emptied. The readout on my tank sensors showed the fresh and black tank empty, but the grey tank full. Thinking that there was grease on my grey water sensors (they are Horst, sent to ETI before ETI was installing them as standard), I filled the grey water tank bu using 4-1gallon jugs and added 2 cups of Dawn dish detergent. I put 26 full gallon jugs down the sink without having any flow up into the shower. I connected the trailer to the truck and hauled it somewhere around 60 miles (100 km) and then drained the grey tank. The readout still showed it to be full. Then I read that you should let the detergent sit overnight. So I repeated the process again, filling and dumping 26 gallons down the sink and added another 2 cups of dawn, again without overflow. I let it sit for 4 days before draining. Once again, the sensor panel indicated it was full. I finally gave up and called ETI and spoke with Dustin in parts. He said that they would have attempted to clean the sensors the same way, and given the results, he was certain it was the sensor panel itself. Unfortunately, ETI had switched panels, but I called the company and a company rep told me the circuit boards do not fail often, but it does happen. I paid $15 for a new board, installed it, and with my fingers crossed, pressed the test button and voila, the grey tank now indicated empty. So glad it was not a wiring problem with the tank probes. However, I am sure that the advertised capacity of my grey tank is accurate at @ 28 US gallons. Can’t say the same for the black and fresh tanks because I haven’t had any reason to check them.