I had a similar problem. I ended up plugging tubing into my overfill tube, and into the tank fill fresh water tube with a bung, and running the tubes into 2 liter bottles. A short trip around the block and the fill tube had sloshed into the bottle about a liter, the overfill tube about 1/4 of a liter.
From then on, I capped the main tube tightly, and because I needed the overfill tube to allow for water extraction, I kept it with the hose above the level, but into the water bottle for the sloshing.
I found if I didn't fill past three quarters, the problem was mitigated.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel