Have you done the one wheel up test to check it?
Are all the tyres identical, same make, same type, with least worn on the rear?
Replace all the vac pipes, as they are known to perish.
Also clean out the inlet manifold of the accumulation of goop. If those don't improve things, then test the MAF by unplugging it. If the power improves, then replace the MAF with a new Bosch item.
The V6 is fine for towing, just as good as the TD4. The only thing you need to remember is keep the engine revs up. The KV6 produces maximum torque at 4,000 Rpm, so it needs to revved, to get the best out of it.
The TD4 has no more torque, it just peeks at a lower Rpm that's all. Oh and uses less fuel too.