Drive fault when towing

[Tractorboy54]

Good morning,

Hopefully this is the right place to post this, apologies if not.

I recently purchased an allroad 2017 2.0tdi and had the towbar fitted last week(independant fitter).

I paid for dedicated electrics and took the caravan out yesterday for first run.

All was fine for 60 miles then I got drive fault error and limp mode.I knew battery was a bit sluggish so went to halfords and alternator tested fine and battery at 620(whatever that is).

Then unhooked van and drove solo for 8 miles with no issues.

Hooked van back on and fault plus limp mode again after 3miles.

Any ideas?

Regards

Paul