I have a 2010 e 350 12 passenger van that came from the factory without the trailer tow package. I wired all of it for seven pin trailer connector and electric brake controller. For the trailer tail lights I tapped into the tail lights on the van. My problem is that when I pull my small trailer that has led lights it flashes normally, but when I pull my travel trailer or any other trailer with incandescent lights it flashes fast. No bulbs are burnt out either. I know that in the past I would of changed the flasher to a heavy duty one but I can't seem to find the flasher on this vehicle, nor do I know if it is a electronic one. Any help would be greatly appreciated. I hate those lights flashing fast.

 

As you probably know the flashing rate is affected by the load (# of bulbs & type) .
Higher current = faster flash rate. The flasher is of the electronic type and is under the dash to the right of the steering column. Most of the time you can remove a piece of plastic dash panel and look and feel where the flashing noise is coming from.
I have found some after market flashers don't work as well as original eq. flashers.
Once you locate it , you might swap flashers and hopefully get one that will give the rate you desire.