We dry camped in Key West and needed to dump every few days.
Our "Blue Boy" was carried on a rig I built over the rear bumper. The photos are pretty self explanatory as to how I built it. The BB is VERY light empty and even with the rack that I built weights almost nothing.
My problem during use was there was almost no room to put the BB under the drain. I had to make a very short sewer hose (about 3 inches long not including the female connectors) to make sure I had a vertical drop.
The vertical drop is important because that way you will use the holding tank vent on the roof to add air to the BB as it fills. (It will "burp" into the holding tanks as they drain and you will get a complete fill without water pooling in the hose)
If you run the hose on the ground and then up into the BB tank filler, then the BB must be vented to the atmosphere at the 3/4 inch hose fitting on the other end. If you do not the BB will pressurize and stop filling. This is stinky and your neighbors will not be happy. You will also need to lift the black water out of the hose to drain it into the BB before you can unhook from the camper. Also stinky work.
I got a 42 gallon BB to drain my 30 gallon holding tanks. My thought was to be able to flush a little soapy water through the pipes even if my black tank was full.
Many times was able to dump both tanks and make just one trip to the dump station. Most times it was a two trip day though. I called it "doing my chores" Hump a 5 gallon water bottle to the fresh water site four times; then two trips to the dump station. (Always in that order!)