Welcome to the group!
Looks like you have a nice bus there that's already been outfitted with all the comforts of home. If I understand the situation correctly, you're not quite ready to do a complete tear-out and rebuild, right? But want to know how to make it comfortable enough to live in for the winter?
One of the easiest and least expensive short-term things for you can do is Ice Czar's #3 suggestion: "zoned heating". This could be as simple as hanging thick blankets between the living and sleeping areas and moving a portable heater into the space with you. Or you could get a little more creative and construct a wall between the "rooms" complete with a door... but, that involves construction which you may not be ready to tackle yet.
The second easiest and least expensive thing for you to do is insulate with a heat reflective material. Since you have shag carpet already glued to the walls, you've already created a small thermal break between the metal and the living space. Unfortunately, shag carpet isn't a great insulator, so you may consider temporarily layering this type of reflective insulation over the carpet to bounce the heat back into the area where you stay the most.
Large rolls of this foil-faced bubble wrap is some of the least expensive insulation that is the easiest to work with and provides a decent amount of insulation for its thickness. Most hardware stores carry this stuff in 4' x 25' to 150' rolls. In the spring or summer, this stuff is great to reflect the hot sun out back out of your windows... and is easy enough to roll up and store out of the way if it's no longer needed. When you are ready to do a complete tear-out and rework, you can recycle this stuff and use it in the walls.
And there's nothing to say you can't combine the two efforts. Since you have a large sleeping area, you may wish to portion off a smaller area that will be easier for your body to heat and double insulate that smaller space. This will allow you to sleep a little bit better... and getting a good rest is very important to how you'll perceive the general comfort of the day. In the summer, open that space up again to allow body heat to dissipate.
One of the most expensive, but in my humble opinion the best overall solution, is to install a wood stove. Several folks have wood stoves that were created specifically for boats. These tiny units are great for busses because they were designed for small spaces that are in constant motion. Information on these stoves may be found here: http://www.marinestove.com/installation_views.htm
Above all, be creative. Something as simple as a fish tank can act as mass thermal storage. In the winter, just make sure to insulate at least four of the six sides (bottom, sides and rear of the tank) to help maintain a healthy temperature for the fish. The potting soil in house plants can also act as thermal storage. Again, make sure you insulate the body of the pot to help it maintain temperature.
Have fun, and good luck!
P.S. - Don't be afraid to post photos of the interior of the bus. We're all curious skoolies here.