Now I just got this information from etrailer
Your 2018 Forester motorhome is built on a 2017 Ford E-450 chassis so the correct brake controller install harness is the part # 3034-P for a Tekonsha controller like our best selling Prodigy P3 part # 90195.
For a brake controller I would recommend the Prodigy P3 Brake Controller part # 90195. This is our best selling brake controller because of how well it works and how easy it is to setup. This is a proportional brake controller that senses the amount of braking the tow vehicle is applying and applies a proportionate amount to the trailer brakes. This will be the last brake controller you buy. It can easily be transferred to future vehicles as well.