Ahh the dreaded toilet hiss. Had the same thing after messing with the fittings in the adjoining cabinet to tighten them. Look at the diagram Lee posted. Mine cracked at the plastic fitting at the rear of part 8 - guess I was a little rough on the toilet feed line in the cabinet.
Push down on the pedal and put a plastic soda bottle in the toilet hole to hold it open. Water off to the toilet of course. Next to the pedal is the front hold down bolt head. Remove the plastic part number 11 and look in to see the second hold down bolt. That one is a bear to get out. I highly recommend you get your bolt size from the front one - then go to Lowes and get one of their flex head ratchet wrenches.
GearWrench at Lowe's: 16-Piece SAE Combination Wrench Set
Once the bolts were out I couldn't get the copper feed off without fear of breaking the rest of part 8. So I cut the line at the back of the toilet. The fitting on ours at the rear was in two parts. So we were able to replace the broken piece with copper. Hopefully you'll find that is the only part that cracked. I replaced the copper toilet feed line with 3/8ths by 3 feet stainless line and a step down coupler on both ends (Lowes or Home Depot). If you pull the toilet you'll need the gasket in part 10 - get that from almost any RV Dealer.