Keeping the GMC warm or cool can be a challenging problem. There's a lot of glass in the coach - that makes it great to drive and look at scenery, but it also means that solar gain can be a problem. The coach walls and ceiling have blown-in urethane insulation in varying thickness. The tightly adhered foam insulation reduces "sweating" when the outside temperature is low enough to condense moist interior air.
But with as much thought as the GMC engineers put into the coach, the insulation still isn't as good as it could be. So, Irene has taken on the challenge of improving our insulation. Her first stop was to Camping World for several rolls of Reflectix mylar insulation. She then made shades that could be attached to each window with velcro. Now, when we boondock, it's a simple matter to put the Reflectix insulation in the windows towards the sun (at least those that we don't mind blocking) and reduce the solar gain tremendously. She made covers for all of the windows, so when the coach is stored outside we can cover all of the windows. This provides shade and security - people can't easily look inside the coach. RV stores sell pre-made window insulation but at a very high price. We saw small window insulation pieces for $30-40 each. Irene made covers for all of our windows using two rolls of Reflectix for less than $40 total.
We're also using Reflectix as additional wall and ceiling insulation. Most locations in the coach have enough room for the 1/4" material to fit between the existing insulation and headliner material. Pieces are cut to fit snugly into the space between structural members. Pressure on the insulation keeps it in place while we work. Here's aview towards the front of the GMC. The view towards the back, curbside. And the other side of the GMC looking back.
Irene uses aluminum tape to join pieces of Reflectix together. Depending on the application, Irene usually tapes just one side of the panel. Where she is concerned about air infiltration she tapes both sides. Most RV stores carry the tape for use with Reflectix. Or, you may be able to save a bit by purchasing it at a home store.
Reflectix varies in price tremendously depending on the store. We've found that Camping World has the lowest prices in our area (~1.95/ft) compared to $3.95/ft at another discount RV store.
We're also using a urethane foam insulation to close gaps and create air stops where needed. We paid about $13 for a spray can that is advertised to provide 3000 cubic inches of foam (this will fill a small book box cardboard container). You can use the can in stages - just cut off the last inch of the applicator tube when you're ready to continue using the foam. The foam hardens to a workable form in a few hours. It can be cut and shaped fairly easily.