How can I make FL Mobile Home warmer on cold days without major work?

Insulate the underside of your trailer

I don't know what you consider major work but I put plastic over my windows and that really helps and I have dual pane windows. I use black plastic on a couple of windows that get extra sun so they absorb more heat.

Hi Linda! The easiest would be close up any gaps (spray foam is easy to use) and hang insulated window coverings. Gaps and glass cause a lot of heat loss.

Check the weather stripping around your doors and windows. The door sweep at the bottom gets worn down and is replaceable. There is also a vinyl/rubber weather stripping on the door frame that gets brittle as it gets older and can crack. This is also replaceable. Older homes may have foam weatherstripping that may only last 3 to 5 years. If yours has gaps or nicks remove and replace it with new weatherstripping. Here is a guide to the various types of weatherstripping. https://www.thisoldhouse.com/ideas/essential-guide-to-weatherstripping

Windows are the main area of heat loss in any home. You can add foam weatherstripping to windows to seal small gaps. https://www.houselogic.com/organize-maintain/diy-repair/how-to-fix-drafty-windows/ A better way to insulate windows is to add another layer that creates a dead-air space between the window and the interior. Shrink Film kits are sold for this, but they are only good until you decide to open the window. There are companies that build and sell window inserts. Though these are intended for traditional homes, a modern manufactured home will likely have the type of window this will work with.

Installing traditional batt insulation on the underside of your home provides warmer floors, but you must also add a layer of screen (hardward cloth) to protect it from rodents.

For $8 you can get a highly reflective 'curtain' to hang in the window. I have used them all summer and they reduced the heat in my bedroom tremendously. I am leaving them up for winter. They are just mirror like mylar. You could probably just get some mylar from a craft store and use liquid starch and 'wallpaper' the material to the window. That's what I did with aluminum foil before I got these 'curtains'. I also have blinds that hide the mylar curtains and offer another layer of protection. Or...there is an article on this site for insults-bright and how to use it to totally insulate the windows. I too live in a mfg. home.

In terms of heat loss, these ideas are great. If the issue is just keeping the area you are in warmer, a quartz heater with blower might be a good idea. No need to crank up the heat for the whole house when you are in one room.

I would also invest in insulted drapes...keeping heat out in summer, keeping heat in in winter and cutting any drafts

Thank you all for your answers I will be trying some of them.