I would say "osewa ni narimasu. yoroshiku onegaishimasu." when I meet my host family for the first time. And I'd say "osewa ni narimashita (Thank you for taking good care of me). iroiro arigatou gozaimashita.(thank you for everything.)" and add some comments about my stay with them like "totemo tanoshikatta desu.(it was a lot of fun.)" when leaving there. I hope you have a wonderful time in Japan.

Just repeating what I've heard - don't use keigo, they prefer you to speak to them casually. I guess you're one of the family while you're staying there so to speak. I also saw a video of a girl who tried to help out with her family setting the table and well she didn't do it like they do it in Japan and then felt awkward when they fixed it for her. Anyway, good luck!