We expect hosts to provide a free parking space but not incur costs. Many of our hosts don't actually request payment, but if a guest uses hookups they must offer to compensate the host for any costs. This is part of the Courtesy Contract all guests have agreed to abide by.
If the host doesn't specify an amount, in most cases offering $3 to $5 per night for electricity seems fair, but it will vary depending on how many services are used (electric, water, dump, WIFI, campfire wood, etc), and also the size of RV and whether a/c or furnace was used.
Hosts who expect to be compensated a particular amount should mention it in the House Rules section of their profile. To ensure there's no misunderstanding, they may want to remind the guest again and specify the amount when they accept a stay request.