You must report your gross winnings as taxable.
You can deduct your losses up to the amount of your winnings. This is an itemized deduction so the actual benefit will depend on your other itemized deductions and overall tax situation.
If audited, you need to prove your losses. I'd guess you lost more than what's on the statement, but can you prove it? You might have ATM withdrawals showing money spent at the casino or racetrack. If you want to know how the casino calculated your wins and losses for your statement, you will have to ask them.
From the figures you mention, it looks to me like you need to report $18,000 from the W-2G, and then you can report $13,000 of losses (because the casino says your net win is $5000). If you can prove you lost more than $13,000, you can claim what you can prove, up to the amount of your winnings.
