Can I stay with my to be husband outside the US as long as my re-entry permit lasts?

It is possible but will delay your eligibility to become a U.S. citizen. I recommend you get his I-130 on file ASAP if you haven't already. You also need to keep in mind the dangers of abandoning your own permanent residency even with a Reentry Permit so be sure to pay taxes every year in the U.S., maintain a residence, bank account and other proof that you will return as it is a factual decision by the Port of Entry Officer or consular officer when your spouse is able to come to the U.S. whether you still intend to make the U.S. your home.

Your re-entry permit allows you to remain outside the U.S. for the validity of the permit. However, if you remain outside the U.S. for more than one year, even with a 2 year Re-entry Permit, it breaks the continuously residence needed for Naturalization and you must wait 4 more years to file for Naturalization. Also, if the Priority Date becomes current while you are outside the U.S. you have to show you are domiciled in the U.S. for purposes of the Affidavit of Support. Even if you have a co-Sponsor, you as the Petitioner/Sponsor must prove you are domiciled in the US when your husband is interviewed.