Well what are you doing? "Covering" an event can mean very different things. Are you coving it for a magazine, Mopar, Jeep, a newspaper or for the event organizer? Are you there to provide prints on location for sale?
Well figure out what you should make each day and, have some cards with contact info. That way you can charge any owners of the jeeps who want prints of theirs in action. For me I would use my regualr jobs hourly rate + the OT rate. THey should be covering food and, lodging as well. Also you dont want to give away your rights to the photos either.
You might want to talk to the customer/magazine and see if they want a particular area covered. At jeep events there is always one tough spot that lends itself to great shots of the Jeeps and the driver working the trail and how you will get there if it is a ways into the trail. You might have to hitch a ride in a jeep. Other than that on pricing I will have to defer as I do not have any idea on pricing for this type of event.
As you don't know what to charge then I'll guess that you don't do this often enough to charge what the 'big boys' do but you really don't want to do work for hire.
Work for hire means (for any that don't know) that you agree for the wages you are paid you give complete ownership to whomever paid you. So if you have a killer shot or twelve that the jeep owners want in a 16x20 canvas wrap and you sell them each one- you have then committed copyright infringement and could be sued back to the stone age. Or in other words, any image making you do there after would be of the burnt stick on a rock wall variety.
Since you are talking about two 14 hour days add another three of those (yes, count the post processing) for a total of around 70 hours times around $50 per hour or whatever you think you're worth. Add any travel expenses, additional insurance on your gear, house sitters or whatever expense you will have to pay that you otherwise wouldn't and then multiply all of this by 1.15 and save the 15% back for gear replacement or new purchases. You could get into rights per year at x times viewership but that gets complicated really quickly.
I wouldn't do per diem because they could then leave you to your own devices to get your food which could be time consuming -time which would be better used shooting. Try getting them to provide food, beverages and event transportation (starting and stopping at the airport if you have to fly in).
Mike has made a Lot of good points, arangeing for them to provide food for you might work out better considering the schedule. Make sure all of this and what uses of you images, who retains copyright, and in what format the product is deliver, is all clearly defined in a contract.