Leave the slides in and you'll have less volume to heat. They'll probably seal better when they're closed, also.
You should leave open any cabinets that have plumbing inside so the heat can keep it from freezing.
Cover windows. Here's one way that may work, depending on your window treatments. Instructions here:
http://rv.tothewoods.net/InsulatingWindows.htmlSame deal for the windshield...you'll lose a lot of heat to all that glass. We isolated our cab with these curtains, that can be pulled back during the day. Instructions:
http://www.rv.tothewoods.net/CabCurtains.htmlMoisture will always be a problem. You can help a little by cracking a vent or getting a dehumidifier. One thing to keep in mind...you're only seeing part of the condensation. There will be more inside the walls.
Putting a rug down can help insulate the floors.
You can get small electric heaters, too. I use a
Pelonis Classic Ceramic Heater...a little bit spendy, but it has good out put and it's safe. Just be careful about overloading your RV's electrical system.