I would take the control apart and look!
There is a control valve at the rear of the toilet, where the water line connects, that the front pedal actuates. Likely you have a bad seal in there. Before you go out and buy a new toilet, take the old one off and investigate the innards of that control. (you will have to take it off to replace it anyway)
It could be something a simple as an o-ring, but it shouldn't be too hard to see just what stops the water in there. You will probably get more informative answers from those that will know where you can get rebuild kits for the various toilets.
Look here http://rv-roadtrips.thefuntimesguide...let_repair.php