If you get a rolling machine tote big enough you can slip an 18 x 24 mat in it on the back side. Several rulers will slid behind the mat too. I normally take three rolling totes to a retreat, my machine bag with the machine, cutting mat and rulers, a rolling milk crate with Art Bins with my projects, and a small rolling tote with every thing else. I also take one rolling suitcase with my clothes.
So far one of the things I've never needed at a retreat is an iron. When I go with my guild one of the members with a van always stops and picks up the guilds boards and irons. You might check with the retreat organizers to see if you actually need it. I know our location restricts the number of irons we can have plugged in to prevent blown fuses.
Some extras that I throw in my small tote are my big power strip, regular power strips and extension cords and a couple of three prong to two prong adapters.