We have a static lodge/caravan, and I wouldn't dream of charging friends who come to stay. There are no petrol costs, but we are already paying the rent, the electricity, gas, insurance and so on, but having other people to stay only adds a little bit to what it would cost us anyway. I would accept an offer of halves on food and drink if they are staying longer than a weekend, but it wouldn't cross my mind to profit on the stay.
When people borrow it for their own use, people usually leave a bottle of wine or something. It wouldn't cover the gas and electricity, particularly in colder weather, but I wouldn't take money from a friend. I have known people to take a lend, though - including a cousin who used it regularly, but ate all our food, used up the electricity (we were on a 'card' system then), drank the special wine I'd bought for my husband, and left us to come back to total darkness and no loo roll, as he'd used the last of that, too. I just stopped letting him use it, by telling him that it was busy every time he asked.
It was partly my fault, I think, for not being up front about being annoyed at his behaviour, though. If your friends have asked you to contribute (and sharing the petrol costs is only fair), I agree with those who say that you should ask how much they want before agreeing to go. If either 'side' is uncomfortable with the arrangement, it could put a strain on the holiday, so it's better to be clear from the start.
I don't think it matters who has more means - that doesn't alter the costs, but if that is the case, there is no reason why you shouldn't pay for a weekly evening out as a treat.