If your black and grey share a common “exit” from the rv into the sewer dump hose, you can equalize your tanks while boondocking. Grab an extra valve:
Valterra T58 Twist-On Waste Valve https://www.amazon.ca/dp/B000BGHYJS/ref=cm_sw_r_cp_api_glt_i_08FE8WD814635QZ8JK9P?_enc oding=UTF8&psc=1
and install it where your sewer hose would connect. Make sure it’s closed, then open your black and grey valves. Whichever tank is fullest will flow into the other tank until they are equal in volume. So instead of being limited by your 30gal black, or your 30 gal grey, you are now limited by your 60gal everything tank.
Some people find it disgusting to think of black water in the grey tank, but as far as I’m concerned, they are both pretty foul.
I would keep the black and grey valves closed except for while you are equalizing the tanks. When it’s time to dump, open your new third valve, then dump as normal.