Do you mean Ford Fiesta? I haven’t seen a Festiva for a long time.
My past experiece towing things behind a class C motorhome toaught me that it really doesn’t matter what it is when it comes to the motorhome’s fuel economy. Whether towing a rectangular cargo trailer, a little Ford Fiesta, or a 20 foot boat, the more important factor in the motorhome’s efficiency is all the square feet of frontal area on the motorhome itself. In fact, I have suspicions that the motorhome mileage is actually slightly better when something is towed behind it. Maybe it has to do with how the air closes back in behind the whole rig.
To answer your question, I think you should just tow along whatever it is you want, because I don’t think the choice will make much difference to the motorhome, as long as you stay within its weight ratings.
2020 Sunseeker 2440DS on 2019 Ford E-450, Trekker cap, Topaz paint