Fix Leaking RV Toilet

Be sure that the blade valve is closing all the way and does not have  paper jammed in the slot it closes into. If the blade is not seated, the valve will run water. If that is not the problem, try putting your foot under the pedal and gently lift up. If the valve closes and water turns off, it is probably the spring that may require some lubrication. If it doesn't turn off it is probably a faulty valve. There is one other item that comes to mind and that is the Vacuum breaker. It has been a long time but I think that also can cause leaking into the bowl. I am going to check that out.
Here is a link to Thetford FAQ's

Turn off the water to your rig  so you can depress the pedal without filling your tank. When you depress the pedal, get down low and look inside with a flashlight behind the pedal. You will see the mechanism/spring. While holding the pedal down spray some lube on the moving parts. It is probably better to use silicon spray instead of WD40 as silicon will not attract as much dust.

Installing toilet?

I have a flat 1/2" ratchet wrench kind of like a box wrench which makes the job pretty easy. I can have the toilet out in about 10 minutes. I reach around behind the toilet and feel for the nut. Put the ratchet on it and loosen the nut. Then I can usually turn it off by hand. For the front it is necessary to depress the pedals (water supply / with pump turned off) so you can get the ratchet over the nut. You can put (wedge) something in the hole to keep the blade valve open which will keep the pedals out of the way. I have used a spray can with a string securely tied to it just in case it fell thru, which has never happened. The front can also usually be turned off by hand once loosened. Before you start be sure to have a new "doughnut",the rubber flange gasket, for between the toilet and flange. It is also not a bad idea to have a new fitting that attaches the plumbing to the toilet just in case it gets stripped or damaged in any way. If you have the space, you might want to put in a shutoff valve where the water line goes into the toilet. This is cheap and if you have to work on the toilet again, you won't have to turn off all water to the trailer. Check out any fittings you might need and buy them before you start. Anything you don't use can be returned. If you need it and don't have it, you will not be a happy camper. It has taken me longer to write this than to change out a toilet. I have performed that job many times over the past 30 years. Be sure to carefully line up and thread the coupling, it is easy to cross thread. Another thing is that the coupling doesn't need to be much more than hand tight. If it is leaking, tighten a bit more. Oh I almost forgot, putting the toilet back on is basically a reversal. Line up the two bolts and the toilet and just ease the toilet down on them. Get the nuts started and just snug them a bit then check for orientation of the toilet. When it is straight, tighten a bit at a time front and back so you compress the gasket evenly. You will do that before making the plumbing connection. Good luck

Bad odor from the toilet?

Try cleaning the slot that the blade slides into when you release the pedal. There are two bolts that secure the toilet. One behind the pedals and one behind the lower left corner (facing the toilet). Try tightening those to compress the seal. Do not over tighten just snug them up. Try that BEFORE you go to the trouble of removing the toilet. As for the floor seal, they are pretty standard and easy to replace.