We have the same toilet, I believe, in our Country Coach. There is a small cupboard above the toilet where I keep bathroom supplies. The cupboard wall leaves a small space about 5" wide between the cupboard wall and the coach wall. Inside there is a small dial that adjusts how much water the toilet adds to itself with each flush. Does your toilet have anything like that? I've attached a scanned copy of the second page of the manual that someone from IRV2 sent me! PM me if you can't read it and I can email the whole thing to you.
John & Cathy Lamb
2022 Durango KZ