I haven't done it yet, but I plan to. You will need 12v for power, antenna for radio and wires out for speakers. Note that some RVs may not have their existing speakers wired for stereo, but rather as mono. In that case I would try tying better wire to the existing wires and pulling them through. If this didn't work, and I couldn't snake wires through using a wire snake, I'd probably run them in a channel right up against the roof, perhaps hidden behind moulding.
The most difficult part was the cabinet work. My trailer ('98 Wilderness) came with a crummy and oddly shaped stereo unit that left a hole larger than the one used by regular car stereos. I ended up buying a cabinet door from a surplus place (a good match for the other doors in the trailer), glued it over the hole and cut a new hole in the appropriate shape.
Wiring was the same as with the automotive unit. The only modification I had to make was the installation of a shut-off switch for the stereo. All of the new stereos have instant-on, clocks and memories that draw power even when the unit is turned off. The amount of power used is not great, but is a consideration if you do a lot of dry camping.
I also installed an "antenna booster" at the same time. I always they were little more than snake oil, but the darn thing actually works. $9 on E-Bay and I can bring in dozens of stations, even in remote areas. Turn off the booster in these areas, and all I get is static.