I've seen this too on mine. I put the hose in the tank, it comes out like it's full. I look at the tank gauge and it says 1/3. I stuff the hose in the tank and hold it there until I see water coming out the over flow. I pull the hose out and the water shoots out of the tank for a bit and then a big blast of air comes out. Tank now 2/3 full. I do it again and the same story. If I fill at the CG with jerry cans, same story. Ergo, my conclusion is, that the tank vent line was missinstalled at FR and not able to let the air out of the tank at the same rate the water is going into it. It's not totally blocked but crimped. Ill bet that the tank is getting bulged out when the water is going in fast and wants to push the water out when you remove the flow into it. Mine's at the dealer at this time getting the last few warranty things corrected before winter... I hope!
2016 Flagstaff 27VRL Emerald
2014 GMC 2500HD CC 4X4 6.0L