@OP -

Since you have not admitted to any guilt, why not have your girlfriend call up Blizzard? If the addresses between the two accounts are different, you could argue that your girlfriend lives (and plays, no pun intended) with you. Also, depending on the extent of Blizzard's IP tracking, they should be able to verify that both accounts were always/almost always logged in with the same IP address. I must admit though that your story begs the question: where did you get 500g at level 60 from? I've R-A-F'd plenty of characters, and I'm lucky to have 100g at 60, even while R-A-Fing with only quests. If you got it through some legitimate source (Auction House, item trade, (please don't tell me that you farmed 500g at lvl 60 :P ), or whatever), Blizzard can easily verify that, and I'm sure they would be more than happy to assist you.

The economy exploitation letter is pretty generic, but is usually triggered by your character giving out gold to multiple sets of people. One or two transactions typically doesn't set off a red flag, but 5-10 would.

EDIT: Also, you claimed that you didn't buy/sell gold, or bot... but did you pay a powerleveling service to R-A-F you? Many of those companies will use your characters as toons to trade gold with. They will also create level 1's to spam trade channels with gold advertisments (what did you expect?). If you're absolutely certain that you didn't engage in any illicit (as deemed by Blizzard) activities, you shouldn't have a problem getting your accounts back.

Good luck.