Here's my helpful guide for anyone wondering how to replace the distributor on a TBI 454 in a Fleetwood Southwind in the rain.
First, remove the distributor cap:
Now mark the position of the rotor, and make a line that lines up with the lip where the floor ends and the engine bay begins:
Looking at the first image again, can you see the distributor hold-down bolt? Of course you can't, it's hidden under the fuel lines and nearly impossible to access! Channel Winnebago Man and spew colorful words at both Chevrolet and Fleetwood's engineers as you painstakingly loosen it 1/12th of a turn at a time while your face is inches from 23 year old motor home carpet.
Done? Good, try not to drop the bolt or the dumb little clamp thingy. Now unplug the distributor, and have a brief moment of terror as you realize that it looks like the floor may prevent its' escape. Pull it up until the rotor stops turning and mark that position too.
Now lift the distributor, tilting it slightly forward as you reach the floor, breathe a sigh of relief that it clears, and say something nice about the guys at Fleetwood for the first and only time. Transfer your marks to the new distributor:
Panic yet again as you remember that the internet said the oil pump drive thingy always moves and berkeleys everything up. Shine a light down there and decide it seems fine:
Use your reference marks to align the new distributor and rotor, say the traditional Haynes manual prayer "installation is the reverse of removal," and stick it in the hole. If it worked then the housing is lined up with the floor just like the old one, and the rotor is pointing where it should:
Remember your old friend the hold down clamp? Put that thing back on- go ahead. Try. Spend ten minutes or so fighting it while unleashing a continuous stream of expletives that would make Winnebago Man blush, eventually getting it by sheer luck. Make sure everything lines up. Now snug it down, transfer your plug wires to the new cap, and install it:
Find and unplug this stupid connector to disable the advance mechanism:
Now put on some clothes you don't care about, and wonder whether your timing light is waterproof, because you're going here:
Hook the timing light's sensor to the #5 spark plug wire, and find this rusty thing on the underside of the engine:
Get really up close and personal with it, so that you can hopefully see that the big "V" is zero, and one of those marks above it is 4 degrees BTDC, which is what you're aiming for in terms of timing. Start the engine, and rejoice momentarily that it actually runs before worrying about how awful it sounds. Get back down in the mud and see what the timing is.
Now get up again, and go turn the distributor couterclockwise because your timing is hella retarded. Ruin the interior of your RV on the way there, and enjoy how being soaking wet makes you extra conductive and therefore easily shockable by way of distributor. Do this several more times until the timing is what it's supposed to be.
Now, shut the engine off, and tighten the stupid awful E36 M3ty hold down bolt that some drunk shiny happy person at Chevy designed on the worst Monday of his life. Check the timing again, hate yourself because tightening the clamp advanced it one degree, and do nothing about it because it has knock sensors so it'll probably be fine. Reconnect the dumb wire.
Your clothes now look like this, go change before you test drive it:
The thing started hesitating on the longer uphill sections, not dying but occasionally popping through the exhaust and slowing down more than I'd like. 
