Well, we are planning a much cheaper trip this summer, but same idea :) We had planned a family camping trip with stbx's parents this summer for a week, which is longer than I am comfortable leaving the kids for (plus it is 8 hours away driving), so I said I will go with still if he wants to take the kids. We haven't planned out the finances yet, honestly, but I imagine if I drive separately, I will pay for my gas, etc., plus my portion of the food? I am also taking off work for a week, which is a pretty big financial sacrifice for me (I am self-employed, so no paid vacation). His parents are paying for the campsites, and I will expect him to cover the other costs for the kids (food, outings, travel, etc.) which seems fair to me...?
Perhaps you could ask the same, that he cover the costs for travel, food, lodging for kids and himself and you will cover your own costs?