Jorjan, first let me welcome you to iRV2.com. We are glad to have you join us here and we look forward to reading of your adventures and experiences. Certainly you could have stains under the plastic cowl of the pedastal. You may have to pull the toilet to investigate.
It could also be an open overflow drain that goes around the flap valve. Try this first; to fill the trap back in this drain, just fill the bowl to the top of the inner rim to force water down the over flow.
Do you also have a gray water vent possibly in the same area under a sink? You may have a bad vent valve that is not sealing back also.
Let us know what you find.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7