The dealer is half correct and half stupid as many dealers seem to be about pop ups. The correct part is that FR A-frames have no grey water tank. The stupid part is to say anyone should drain onto the ground. Most campgrounds take a dim view of this, because as you say it fouls the campsite. It is illegal in most state parks. For wilderness camping away from a stream, it can be OK if using biodegradable soap and solids are trapped.
Like dts3204, I use 7 gallon Aquatainers. They are available at Walmart or from Amazon
. They are rugged, stackable, hold water without spilling when sloshed and are liftable when full. They can be dumped into a campground or home commode instead of at a dump station.
The large cap is threaded with a 3/4 FPT. I put a nipple into that to get a 3/4 MHT on the other side and connect a short piece of hose from the drain converter to the tank. The hose cap is attached to the container with fishing leader wire through a very small hole in the cap. Others have modded the top to accept a 1.5" hose to speed up drainage.
Here is one way they can be used.