Your interior control panel should have switch openings marked tank heater with plugs in them. If this is so the next and easiest step would be to remove the panel and see if there are any unused wires behind it. If there are then you can then go down to the converter and look for other like wires and so forth according to your findings.
You might also give Forest River a call and ask if as a matter of course they include the wiring during construction. Good Luck.
Doug, Virginia, Buster & Sam
2014 F150 Max Tow 3.5 Ecoboost
2015 Flagstaff 27RLWS
2005 Road King