What happens if you default and the majority of what you owe is to the IRS?

The IRS acts like any other creditor only with more power. If you go into bankruptcy, your assets, except for those that are protected are sold and your creditors, including the IRS, get the money. After that, if bankruptcy wipes your record clean, that includes the IRS. There may be other considerations. You will need to discuss the options with your lawyer.