It does not matter if you use a trucking accountant or someone who knows business taxpreping at this point. Even Turbotax will help you straighten out the mess. You need to understand the difference between using the vehicle for business and pleasure, in any case, if the van is used for this work, then it is used for this work only and everything (mileage, expenses, etc. ... is a write off)
Per Diem rates are based on your days away from home, so in a theoritical case, being on the road for 12 weeks gives you 84 days on the road and that means you can take $3946 claimed on your taxes OR (84*$59)*.8 whether or not you spent that much money. NOW IF you spent more, too bad because the IRS will tell you to take the per diem rate any ways.
The mileage/van expenses thing can be either way for a van, but you can't change it once you decide which way to go. If you take the expense way, then every receipt for oil changes, tires and so on have to be accounted for but the mileage way, it is simple - starting mileage at the beginning of the year minus the mileage at the end of the year times the mileage rate (what ever it is). It should cover all the maintenance and fuel for the van and then some.
The problem with Audits is no one wins, it is something that you go into to limit the amount of damage that they can do, as Moot said "Its their game, their rules and open to their interpretation."