I would think the only way the marker lights could be on is for them to have a direct connection to the batteries. You could do some checking as to the wiring--there should be a junction box that the power cable from the truck connector goes to--that is where you could start. There also could be a misconnect within the power cable plug--is it the molded-on plug, or a removable plug. Use of a 12 volt test light will be helpful in tracing the power at different points in the system.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired