Update on this! I changed my text editor to Atto and the forum emails are now in a sans serif font!
We're using Atto and it is still serif... looking for resolution on this as well.
Figured out that one way to do this is to add inline CSS style (style="font-family:sans-serif; font-size:15px;") to templates
/moodle/mod/forum/templates/forum_post_email_htmlemail.mustache
/moodle/mod/forum/templates/forum_post_email_htmlemail_body.mustache
and those templates should probably be copied into moodle/theme/[theme]/templates/mod_forum/ so that they are not lost with an upgrade.
/moodle/mod/forum/templates/forum_post_email_htmlemail.mustache
/moodle/mod/forum/templates/forum_post_email_htmlemail_body.mustache
and those templates should probably be copied into moodle/theme/[theme]/templates/mod_forum/ so that they are not lost with an upgrade.