You'll find that those supposed "window units" on the smaller RVs are actually "sleeve units," aka "through-the-wall." Similar in appearance, but designed to be buried in a wall so only the rear is visible externally. No protuberances.
If you tried to bury a window unit in a wall like that, it would not work. Window units draw outside air in to cool the condenser coils from the sides and top, and blow out the back (I might have that reversed, but suffice it to say the sides need to be open).
Sleeve units are hard to find and much more expensive than consumer grade window units. They are also not as efficient. Plus you lose all that cabinet space, which is at a premium in a TC to begin with.
A window unit is fine, if you have a window to spare. Has to be a rear window because an AC unit sticking out a side window will get knocked off. Also keep in mind that the camper wall may not hold up to the weight of the AC unit bouncing against it going down the road. It also looks a bit "redneck" but if that doesn't bother you, knock yourself out.