For welfare reasons, I think it would be better if they all went together for slaughter. Taking each one individually will be much more stressful for the pig - and the last one will have to live alone which is never a good idea.
You might be able to borrow a trailer from a local farmer or smallholder. If you can get it well in advance, you can start getting them used to it. Having said that, we borrow a trailer so the first time we load them, is the first time they have seen a trailer.
We build a race from the field gate to the trailer ramp, using hurdles, hen arks and the patio table on its side. THis keeps them focussed. We put plenty straw in the trailer and on the ramp; we don't feed them on the morning and we train them to follow a bucket from the time we get them. Finally, give yourself plenty time. If you try and rush them and YOU get stressed, they will know and react badly.
Also, don't have your trailer touching the electric fence so that the first pig to touch the trailer gets a shock!