In practice, I drain my bathroom/shower Gray Tank about every 2 days - assuming we each take a shower a day.
Gray Tank 1 for my kitchen sink, I can go up to a week - just depends on usage.
Probably pretty similar to what you are experiencing.
There are RVIA standards and practices that govern RV plumbing systems. It is quite possible that they preclude tanks being plumbed together. Not sure.
What some people do is a manual equalization of the 2 gray tanks. I am not here to advocate the practice but I have read where some who are boondocking are able to extend their time between tank dumps by making sure the sewer discharge cap is on and secure, then pulling both gray tank valves. This allows the water in both tanks to equalize (in theory). I have never tried it and again, I am not advocating, just re-telling what I have read. Be sure to close those valves before traveling to the dump station and then drain the black tank first, followed by the grays as usual.
Jim