Lack of power when towing

I bought my Ford Maverick 2.7 GLX to tow my caravan. It drives ok when not towing but when coupled up to the caravan it has real trouble pulling it. At first I thought it was the turbo but had that checked and adjusted and this helped a little but still got the problem. Went to a diesel engine company who without looking at is said it was the injector pump and wanted to charge £450.00 to take it out and service it.

Im a bit apprehensive about paying £450 without checking out if it could be anything else. Has anyone else experienced similar problems?