Most TCs don't have a place for a window unit to be hanging out, except the side, and the problems with that should be obvious.
If you think the rooftop AC sticks out like a sore thumb, a window unit hanging out the side or back like a giant pimple isn't going to make you any happier.
I'm thinking the "many RVs come with window units" you're actually seeing SLEEVE units, which have special ventilation porting to allow them to be embedded in the wall.
Even so I can't see 5000BTUs keeping up with a larger TC. 5000BTUs takes 4 HOURS to bring my well-insulated 8x12 bedroom from 79 to 72 running continuously. A TC is far less well insulated. Plus its pretty loud.
