First step is to make sure the water to the RV is off. Next let pressure off the lines by flushing the toilet. At the bottom of the toilet there is a plastic cover that can be removed by pulling it out sometimes slightly up, some models have one on each side, other models have just one that wraps all the way around remove it.
You should now see a bolt on a screw, unscrew this bolt. There is one on both sides unscrew both of them. Don’t lift toilet yet. In behind the toilet there will be a hose connected to the top back of toilet. You should be able to unscrew this, sometimes it is only finger tight. Unscrew this but put a towel down to catch the little water that still may be in the hose. Remove toilet from floor do this carefully to make sure that the bolts stay in the floor (sometimes the toilet will be stuck from being attached so long gently rock it.) PULL STRAIGHT UP.
Get new toilet on the bottom ring that attaches to floor there will be a rubber seal kind of cone shaped and make sure the new one is in place (this prevents sewage from leaking into the floor). Place the toilet on floor making sure the bolts go though the holes in the toilet. Replace the nuts on the bolts and tighten evenly and making sure they are snug and tight to prevent leaks.
Attach hose to the back of toilet snug with fingers first and then about 1/4 of a turn with a pliers. MAKE SURE NOT TOO TIGHT or you will stripe the connection and that is a bigger job because RV hoses are hard to replace and not have any leaks. Turn the water on and check for leaks and if any make sure the bolts are tight and the hose on the back is tight. Then replace plastic cover(s) for looks. Make sure to clean up any drips of sewage with cleaners.