When I bought a used Lance 500 the floor of the shower pan had a small leak around the toilet , the toilet is on a raised part of the shower floor . Thats probably what saved it from being much worse as far as damage .
On mine if you pull the toilet out theres a regular household type toilet flange that the toilet bolts down to , my leak was between the flange and the shower floor . I put a hose at this conection to confirm the leak , I could see under the shower floor from the compartment dr for the drain valves . You have somekind of acsess to that area because the shower drain Pee trap is right there also.
There are 3 or 4 screws holding the flange down , loosen them and seperate the flange from the fiberglass floor and squeese some sealant in ,tighten it down and run a bead of sealant around the flange , worked on mine. Good luck