Hi L.,
I had the same trouble with my son using toilets outside the home. I carried a screw-top bottle and I take him to the restroom to use it. I can then empty into the potty and flush. It's an individual sized plastic juice bottle and has a wider opening than the water bottles. That worked quite well, and he has gradually become more able to use restrooms. He did want to use it at home, I was very firm about the fact that it was only for outside the house, and he once peed on the floor when I said no to using the bottle. After that, no problems I also have a cushioned folding seat called Cushie Traveler
http://pottytrainingsolutions.com/product_info.php?cPath=...
I thought would be good for travel. He took to it and it's what he uses at home (for BMs). I bought a pack of disposable seats called Zeets. (folding cardboard that covers the whole seat, great if you are worried about germs)
http://pottytrainingsolutions.com/product_info.php?produc...
About wetting, I did not have that problem, but I think I just lucked out. I think the best thing you can do is not shame or punish, but rather describe it as an accident and don't make a big fuss. Also, you could try reminding him that the fun will be waiting for him when he comes back!
All the best,
C.