New Toilet splashes out of back when flushing

We just purchased a new Toy Hauler and took it out this weekend for the first time.
I noticed yesterday that the toilet had some water behind it so I investigated and found that when I release the foot lever, the valve spits water out of the back of the toilet towards the top. I took the seat off and see that there's a valve that has a hole, or vent. This is where the water is coming from.

I did a little research and figured it was too high of water pressure so I filled the holding tank and ran off of the water pump but it does the same thing.

Any suggestions on how I can fix this? The pedal seems a bit stiff, but not overly. When releasing the pedal, it makes a definite suction type close which is when it spits the water. You can't ease your foot off as it pretty much slams shut when within the last couple of inches.