[juanete]

First: Have the holding tank vacumed, flushed and cleaned. Make sure the discharge pump works propperly. second: depending on what toilet you have, some like mine have internal rubber seals like the joker valve on the electric part of the toilet that wear out and allow foul smell and return waste water from the holding tank hose.
Check the sump pump that receives water from the shower and air conditioner. The small bilge pump inside may not be working and you may have water around that area.
Good luck