You best be using at least a 12 gauge extension cord for running a heater and a dehumidifier. It would be very easy to overload most extension cords and that 15 amp outlet with those loads. Overload = heat = FIRE!
Most heaters do not recommend being operated from an extension cord as the wire gauge and plug ends cannot take a high resistance load.
Rick & Debbie; Brandy, Dexter R.I.P., & Fritz R.I.P. (The Doxie "Kids")
2015 Jayco Pinnacle 36RSQS 5'er
2011 Rockwood Windjammer 3001W (Traded)
2013 Chevy Silverado 3500HD LTZ, 6.6L Diesel Dually; B&W Goose Neck and Companion 5'er hitch