I have found most seem to appreciate being planted about a foot away.
My Schizophragmea 'Moonlight' did very well.
Being so vigorous, trumpet vine may need no such concern.
I had an extremely beautiful one, Jersey Peach, and killed it, recently. Not as difficult as I thought.
A climbing rose or clematis is planned for that spot.
I will start each a foot from the trellis.

Be aware that if you plant in in a fence line that divides you from someone else's property, it WILL spread to their property, too.

If I were you, I would plant it about 3 miles from the fence, and hope that it never found the fence.

I hope you outlive your trumpet vine. I planted some along a fence and it ate it, the plants around it, the house, the dog and was heading for my nieghbors !

You can't kill it.