For the next couple years, my family and I are planning on traveling all through Europe with our camper. I am a Croatian (and US) citizen, as well as my kids. My husband is a US citizen with Croatian residency. Will the 90/180 Schengen rule apply to us? We will travel to each country in the Europe but will never stay longer than 3 months in each one. Thanks.
The 90/180 rule does not apply because your husband is a beneficiary of the free movement directive (2004/38/EC) under its article 3 when he travels with you (or to join you). He therefore enjoys the right of entry and the right of residence of up to three months outlined in articles 5 and 6. The only documentary requirement is to have a valid passport.
This means, among other things, that your husband is (perhaps surprisingly) not a "third-country national" for the purpose of border control when he is traveling with you. See the Schengen Borders Code at paragraphs 5(a) and 6 of Article 2:
‘persons enjoying the right of free movement under Union law’ means:
(a) Union citizens within the meaning of Article 20(1) TFEU, and third-country nationals who are members of the family of a Union citizen exercising his or her right to free movement to whom Directive 2004/38/EC of the European Parliament and of the Council ( 1 ) applies;
‘third-country national’ means any person who is not a Union citizen within the meaning of Article 20(1) TFEU and who is not covered by point 5 of this Article;
(Another consequence of the Schengen Borders Code is that you can use the "EU passports" lane together at Schengen border crossings; see Article 10(2), where those lanes are designated for "persons enjoying the right of free movement under Union law" rather than actual EU passport holders.)