It really really sucks to pay for people for a wedding and have them bail last minute. Your friend will probably understand, but it's not guaranteed that doesn't negate the "ouch" part of paying for food.
It sounds like the timeline is:
RSVP --> Car broke down --> Car sold --> Looked for ride --> 3 days before wedding you want to cancel
I think the time to have reached out to your friend and said you can't go was somewhere between the car breaking down and when you decided to sell it, as you knew then that you would have an issue getting to the wedding. Now, you're past the point she had to buy food (if self-catering) or tell the caterer a head count. You already knew about the camping expenses when you RSVPd, so that shouldn't have been a surprise. If you can't go, you can't go, but it seems like a mix of poor planning and bad luck.
I'm sorry to seem harsh here and say suck it up and go rather than the "that's ok" you wanted to hear, but I had three people cancel at my wedding, 2 a few days before and 1 at the wedding itself, and it was really frustrating having to politely tell people "it's ok".
I recently paid a $200x2 change flight fee to attend a high school friend's wedding in San Diego. (They sent the invites out late, a week after we had booked flights for another wedding that same week in the Domenican Republic.) My husband told me in 10 years I'd remember missing the wedding rather than the $400 (plus hotel, plane flight, gift...). So we sucked it up, I paid my first ever change flight fee and we took a redeye (I'm too old for that now!) to make it from wedding 1 to wedding 2. Which will you remember? The $60 for a 2-day car rental (try hotwire or priceline - I've paid as low as $15/20 day) or missing the wedding? Also, ask around - I bet you have friends with camping gear. I have tons of friends with tents to sleeping bags I could borrow.