By Yolan

Step #1: Remove the old fireplace

You may think this step to be a no-brainer but think again. To make the fireplace aesthetically pleasing, you can't have any unsightly screws and such visible. Because these were rather well hidden, I had to pull out the manual to figure out how to remove it. You may have to do the same. If you don't have your manual, just do an internet search and something is likely to come up.
RV Electric Fireplace

RV Electric Fireplace Removed
After removing it, I put steel wool and expanding foam in the holes where the wires came out from the floor to keep rodents out.

Step #2: Lay down support boards

I used a 2x4 cut into 4 pieces the width of the opening. Place them evenly in the space. I didn't attach them to the floor in any way. You can do this if you wish but I didn't believe it to be necessary since I would be securing a board on top of them.
Bookshelf support boards

Step #3: Secure the shelf board and the back boards

I used a 2x8 cut to the length of the opening and secured it to the support boards with, I believe, 3 inch deck screws. Then for the back, I had to cut a piece of plywood in half because I couldn't fit it into the space otherwise. I used 1/4 plywood for the back and secured it with 1 5/8 inch deck screws
Shelf and back boards

I measured out and cut a hole where the outlet was because I wanted to still be able to access it. I have copper tape around it. This is probably not needed but I figured it couldn't hurt and I already had it on hand anyway.

Step #4: Tape up the seams

I used black tape on the seams where the back boards meet. The tape doesn't have to be super strong because the boards are already secured with the deck screws on the sides.
Seams taped up

Step #5: Paper over all of the wood

This is more for asthetics. You don't have to do this part. Since the rest of the encasing is black, I thought it would look nice to have the bookcase be such as well. So I purchased this black self-adhering paper and applied it on the wood. I used a gift card to smooth out bubbles and such to make it look nicer. (sorry for the blurry picture)
Black paper applied

Step #6: Install interior shelves

I wanted to be able to have two shelves of books so I measured out and cut some 2x6's to size and papered them over. Then I angled screwed them to the bottom board and also the sides. I didn't attach it to the back board seeing that is only 1/4" thick. I left an open space on the end for taller books
Interior shelves installed

Step #7: Enjoy!

Tada!

Just a note that I did also put a board up at the top but you cannot see it. It was necessary to keep the wires from the stereo hidden. On the ends of the bookcase, I didn't do much. You can't really see those either. I didn't even paper over the wood on the ends but as you can see, its really not terribly visible so I didn't see any reason to.
Finished bookcase

Tools and Materials Used:

Drill
Circular Saw
Scissors
Measuring Tape
Deck Screws
Plywood
Lumber
Self-adhesive Paper