With a portable unit, I try to get it as far away from the trailer as possible. I've never had an issue, but if I have the space and I'm already running an extension, I try to take advantage of as much distance as possible. You just never can be too safe.
2017 Jay Feather 7 22BHM Baja/Andersen WD
2018 F150 4x4 3.5L Ecoboost Max Tow
2015 MB Sprinter 2500 Passenger 2.1L Diesel
2007 Ram 2500 4x4 Cummins 5.9L G56