I've had some luck doing this in ambulance conversions, where the framing is aluminum and the long flexible bits will drill through that. But I have never tried doing that in a bus, where the crossmembers are steel. I've never tried it because I believe it would be both hard to do and, if you could penetrate, it would leave sharp edges that would quickly abrade the cabling. At least in the ambulance, with 2-inch aluminum tubing, we could drill a 3/4-inch hole and run MC (metal armored flex) through it. Bus ribs are not as thick and they don't always have parallel edges...meaning the hat channel often used has some slope to it...so that would tend to push your drill bit towards the sidewall...assuming you found a suitable metal bit.
I hate to be negative, but I think this is a tough path. Most of us remove walls and ceilings, so we avoid that. But if you find a way to do it, let us know. There may be some instances where it might be handy.
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