I'm disappointed

As others have said book in to a cabin on site, or a nearby Hotel,I wouldnt like the caravan idea either.I was in the US at Halloween when my Daughter lived there I enjoyed seeing all the houses decorated, and little children knocking on the door accompanied by a parent.
You may enjoy Haloween more than you think, give it a try.I think the time you are in Canada seems very short for the lenght of journey, but you are used to it,so I hope you enjoy the time spent with your family.

Check exactly when DS would be available. Cancel flights then rebook. You will be down financially but up on happiness not dealing with the Halloween and camping.

Playing devils advocate.
Do you think your son might prefer you to change your plans and go another time?
Unless you have a fantastic relationship with your son and DIL , wont it be intrusive for them and uncomfortable for you ? to share a caravan.
I cant help thinking that you are making all the effort here, going all the way to Canada which they have obviously known about for some time. Its not like its just down the road.
I think its a bit unreasonable of them.
Its only Halloween.

Lucy2: it wasn't me who suggested Halloween might go on for weeks. That was the OP.
I was astounded to read it!

You might just enjoy it. Give it a go. I would. Don't forget to pack a witch costume. Trick or treating with grandchildren is so much fun x

I think on this occasion go with the flow. I think if you let your son know you are not too keen it will cause bad feeling regardless of the fact he has asked how you feel. You will enjoy seeing your grandchildren having fun doing halloween stuff. This is something you haven't done before including seeing your grandchildren enjoy halloween. Children get bored very easily so you may well find they are in a 'good mood' for the whole of the time. Not a bad thing!!

I'd do anything to be able to see my Son, so would gladly do anything just to be with him.
Sometimes the dread of something turns out to be better than you thought it would be.

I agree with everyone who says to go with the flow and give it your all! You might surprise yourself and have a great time but if you don't enjoy it then at least the family will get the idea and hopefully suggest a visit next time that's more everyone's cup of tea. Start working on your outfit now and maybe plan a few tricks of your own to get your own back!!! I hope you have a great time.. X

You are right missiseff, how often do we spend weeks dreading something, and it doesn’t turn out so bad, I’m sure your family will be happy to have you there

I’m sure it’ll be fun!
Just go along with it you might surprise yourself!