yes, this would be an instant fail if you had the whole level with photonviews on each walls
what you need here is to share the level generation data with another mean, ivy your own server for example.
the master player ( the one who create the room ) is going to generate the level, you shoudl then upload this level generation data to your server with a reference to the room name it relates too, then other players joining this room, will simply get this generation data.
but you should not use Photon for this, you could use a buffered RPC ( which means this rpc call stay in cache for any new player joining to get it), that rpc could have the generation data, but if it's too big, then it will be a problem, rpc data is constrained in size.
So get in touch with the author of this dungon creator to see if there is the possibility to get and set dungon generation using a string or some data that can be stored online somehow.
if not, then this is going to be tricky indeed. Also, if this is your first game in multi player, dynamic scenery is maybe too much, I would first make a project with less features, to get acquainted with all the things you need to learn for MP.