Should petrol money and accommodation be charged?

Maybe person C could pay the bedding fee as a sign of goodwill

I agree. It all sounds a bit nit picky and depressing to me.

Rubbish. If I was the driver there’s no way I’d allow this. I’d prefer to drive and the running costs of the car are negligible. A single journey has no impact on them.

Person A has the costs of maintenance, insurance, vehicle tax , wear and tear etc. and is doing the driving so B and C should actually take care of all the fuel costs.

This sounds an awful situation for person B and if they truly look to save money, I can't see how you think it acceptable to not split costs.
However much I like someone, the thought of spending my few days break away with someone suffering a complete breakdown sounds draining, there is no way this will not affect the enjoyment or dynamics of this break away.

Seems mean minded and uncharitable to charge the extra person. If I were person A or B I wouldn’t take any money from the extra person. I’d expect them to help with paying for food and generally mucking in and looking after the place.

And yes totally unreasonable for B to be expected to subsidise C when they be probably wouldn't have gone on holiday with them in the first place, given the choice. Though it would be very nice if they chose to do so as a favour to A, who seems to have rather a lot to deal with atm.

I feel sorry for Person B, having their planned holiday with A hijacked. Person B should really have been given the option to cancel if A really couldn't have left C alone. Maybe A and C could have reimbursed B and gone together instead.

I feel sorry for Person A, having Person C totally dependent on their presence and essentially holding them "prisoner" in the house. It sounds like Person C isn't in a great place, but they should really reflect on whether the demands they are making of A are reasonable and if there is anything they could do (stay with someone/have someone else visit) so A can still have some freedom to live their life.

If B quibbles about paying a third of £40 instead of £10 per person for bedding you can guarantee that person B is going to want to split everything exactly three ways.
Intrigued how a 4 berth caravan could sleep 3 without making the double bed up!

Person C needs to pay up, that’s the fair way, one third each!

A B and C (A and C are family members)..

Holiday costs £130 (just found real prices)

Current:
A - 65
B - 65
C - 0

Either:
They all pay the same.. this is the fairest C pays A and B £32.50 each..

Or If A wants C to free (as they are connected), A pays B So B is not out of pocket.. or gets C to pay £32.0 to B .. C gets a cheaper holiday subsidised by A.

A pays £97.50 (by giving £32.50 to B)
B pays £63 ( a third of the holiday)
OR
C pays £32.50 to B ( - A is out of pocket.. but doesn't seem to mind.. and C has a bargain holiday)

A is not so bothered about the money, wants C to come .. whereas B -sounds more like they have accepted C will come.. They all get 1 room 1/3 of the bathroom etc (I'd say it was unreasonable to expect B to subsidise C..)

Who invited person C?
that’s very important to know

Person C should split the cost or if person A feels so strongly, cover it for them.

Person B arranged this holiday thinking it was just the two of them. Maybe they don’t even want to have a third person go. Maybe they aren’t fond of third person or maybe they think they would be a bit of a Debbie downer and they’re needing the holiday themselves to wind down.

Why should person B compensate person C, simply because they want to go on holiday?

If my friend suddenly decided to bring her relative along and expect me to fork out I would be annoyed too.

Person C 100% should pay a third and if not should be paying 100% of petrol costs as a compromise.

Is the holiday this weekend? What has happened?

C has to be dc or dp of A and it sounds like a shit show!

I doubt B is happy that C is coming. Holiday was agreed and paid for and adding another person totally changes the dynamics. What else could they do but agree though? A should not have asked and C should not have been so manipulative.
Any decent person would offer to contribute especially having hijacked someone elses 'holiday'. Up to the others to accept or not.

Driving hours to just hang out in a confined space eating food from home sounds pretty grim though and having this extra person who B probably thought they'd gotten away from makes it even worse.

Family relations matter. Person C should offer the third to A&B and A&B can say yes or no individually.

people may like C more or less, but they are coming and need to offer.

and A&B may be in different financial situations. Could make the difference in dinners orders, or another vacation.

Is person C a small child? If so, then I can see why B assumed that they would be coming, and was presumably OK with paying half anyway.

If person C is not a small child, then obviously they should be paying!! But an easy way to sort that out would have been C paying the £40 bedding charge and most / all of the petrol.

Third para of the OP says C lives alone and is struggling. Later OP says A is usually with C at night. OP seems confused.

If I was B I'd bail out of this tbh!! It doesn't seem very fair on B and B will probably feel like a spare part.

C is very unreasonable too and so is A for inviting C at the last minute.

What a mess!

Oh yes!
Hmmm, yet they also say A and C live together.

Which is it, @PinkStarAtNight

Yes of course.

Where did it say C lives alone? The OP said Person C lives with person A

In the third paragraph of the opening post.

All depends on how much we’re talking… perhaps they could pay for couple of the meals out if the accommodation is already paid.

no reason they shouldn’t pay the contribution to fuel though - that’s not an already sunk expense.