Not to be Debbie Downer but nothing you do to the underside of your trailer is going to drastically change your life in terms of heat retention. You're talking about single % gains in anything you do. The only exception is if you plan to keep the trailer in the same spot and want to build skirting around it with insulated walls. And even that won't be drastic.
The fundamental problem with TT is the thin walls/roof and the relatively small amount of cubic feet inside. There just isn't enough space to hold much heat for long. The air exchange is just too quick. I've often joked that my RV is the only place I can be too cold and too warm at the same time :)
On Edit: I guess I should add that it also depends on where you are talking about the 4 seasons. Canada is certainly a lot different than Arizona.