We have the same issue. We usually fill the left side access tank when it's angled down towards the curbside. If I fill it full, it starts to leak when traveling at level.
Does your vent hose have a screen on it. If it's missing, it allows more water to flow out. My tank vent location is also on the side of the tank, near the top, but not on top, and the design helps to promote the leak.
I actually used a plastic hollow wall anchor with silicone to partially "plug" the vent.
There smaller holes in the anchor, not allowing water to flow so easily and I'm careful to fill much slower to not allow it to develop air pockets.
Bob and Better Half
2007 F-250 PSD, SW, CC, LB, 2WD
2006 TF32SS Cross Terrain Toy Hauler (sold)
Reese 16k slider