e350 is bad that's why
Normally this is caused by a weak ground at the left turn signal bulb.
On my 1989 E350 motorhome, I had a weak fuel pump near the fuel filter. I replaced that and the problem went away. The pump in the tank was fine.
Sounds like a weak turn signal flasher
You have a problem with your ground. A weak ground will always cause this type problem.
Assuming you have checked all of the bulbs, sockets and wiring, the signal flasher could be weak or the signal switch is bad.
Most likely a weak turn signal flasher Could also be a bad switch
It means you have weak siganl.
I would suspect a weak ground connection.
Weak signal depending upon the type of signal.
There is no signal in the area you are in.
You may have to buy a new battery, as the current one does not ave the capacity to run both at full power anymore.
I think it might be weak. I am not sure.