Make sure you have a direct swap. Standard over-the-range microwaves are 30 inches wide and vary in height. From someone that has replaced the microwave in three different Big Countrys now, the procedure has been:
1. Unplug the microwave.
2. Feel around the floor of the cabinet above the microwave. You should find two or three flat screwheads (typically about 1/2 inch in diameter).
3. Using a stubby Philips screwdriver, start backing out the screws one by one. Have an assistant hold the microwave up (it will eventually want to swing down).
4. With the final screw out, gently lower the front of the microwave down (it will swing down, hinged on the bottom at the wall where it rests on the wall bracket).
5. With an assistant holding the microwave down at an angle, pull the power cord out through the cabinet until it is free.
6. Holding the microwave at about a 45 degree angle, lift it up off the mounting bracket and carry it away safely.
7. Remove the wall bracket from the wall (unless you are absolutely sure the replacement will fit exactly, which probably won't be the case).
To mount a new microwave/convection oven:
1. Most vented microwaves in RVs use a rear-facing vent outlet. Most microwaves are set up for top-facing air flow but allow you to remove, rotate and replace the fan assembly to change it over to rear-facing venting. Do that first.
2. Follow the instructions on mounting the wall bracket in the correct position (template usually provided). You'll have some trial and error finding studs (aluminum, typically) behind the wall to mount the new bracket. (Note that on two of our three trailers, I had to remove mini blinds from the window to allow for the height of the new mounting bracket.)
3. Test the new microwave position by setting the back/bottom edge of the microwave on the bracket and swing it up towards the cabinet. Make sure it will swing all the way up and level.
4. Use the provided template to determine where the top mounting screws need to go through the cabinet floor. Drill those appropriately.
5. Have an assistant help mount the microwave on the wall bracket and slowly swing it up into place, feeding the power cord through the cabinet floor as you go.
6. With the microwave in place, feed each of the top screws through the cabinet floor and screw them into the top of the microwave. It is recommended you lower the microwave a bit so you can see to guide each screw into the appropriate hole before tightening them all down.
7. Plug in microwave and test.