I don't know about anyone else. But I would not want to stay in our funfunder during the winter.
June 2008, we were camping in the mountains, and the temp dropped below 0 (celcius) and it snowed 6" inches. Our furnace was working overtime to keep us warm and our feet never warmed up that weekend.
One of the problems with lightweight trailers (like the FF), is that manufacturer's don't put extras on them. For example, have you ever actually looked under your trailer. Our floor can't be thicker than 5/8" plywood with no insulation at all. Which would explain why our feet were cold on that June weekend.
Those ideas you have would work, except I'd be spray foaming the whole underneath of the trailer, but then you have one hell of a mess to try and fix anything that goes wrong underneath. You could always skirt the trailer and throw a small heater underneath. That would save you from buying all the extras.
Maybe someone else can chime in with ideas to keep it warm.
20XX 3/4 something TBD
2010 K-Z Spree 318BHS LX