The tanks will equalize in level with a hose connected to the bottom of each tank at the waste fitting.
You would need to get two waste outlet caps with the garden hose connector and put a section of hose between the two drain valves and open them once and a while to equalize the tanks. Leaving them open would work, but would probably eventually leak at the caps. You could lay the hose on the ground. If they are close enough, you could use a washing machine hookup hose, as you will need female connectors on both ends.
You could also make a rigid external levelling pipe using thetford connectors, some abs pipe, and a slip coupling to connect the two tanks together. You would connect each one, then fit them together with the coupling in the middle, The coupling is so you can store it, but a hose will be easier.
You will have some leftover in the hose/pipe to deal with when disconnecting, but if you are dumping nearby, you may be able to leave it connected on the way to the dump station. Installing hooks on the trailer skirt to hold the hose up may be an option. Otherwise closing both valves, and using a bucket to catch the water when you disconnect will work too. Then you dump this in the toilet.
They will level so long as the hose doesn't go above the level of the liquid.