Your problem is probably minor, and when it is found you will no doubt kick yourself for the money already spent.
. . . When I turn the key, the altmeter goes up o 12, but when I continue to turn the key and try to start it the meter goes to zero and I am unable to start the bus, nothing happens, no clicks, no turning of the starter. . . The headlights, wipers and new fuel guage have stopped working. . .
This is your clue. a meter is a fairly sensitive device. If there is a high resistance in the wiring, the voltage will show near normal until there is a draw, and then the resistance will take the bulk of the voltage leaving nothing for the meter or the loads. Think of it as the difference between taking a sip of water from a 100-gallon tank with a straw, and trying to empty the tank with a straw as compared to a 4-inch pipe.
An old no-start trick was to turn on the headlights. If they came on, then try the starter. If they stay bright, it is a starting system problem, if they go out, the starter is connecting the load, and the power is being lost by a shorted starter, a high resistance, or a near-dead battery. The internal resistance in old battery cables or a dying battery would be the first place to look, but you have already replaced them.
The first step, after using the dash meter test and before spending money (sorry), is to put a meter directly across the battery, and then try starting. If it happily stays at 12+ volts, then you have a wiring problem. If that meter also drops to near nothing, then the starter is loading it down, and you have either a bad battery or a shorted-out starter.
Since all the accessories have lost power, I would suspect a loose or burned feed to them, or a failed accessory-power solenoid. A little power is getting to the key, as noted on the instrument panel meter, but it sounds like it is not enough to activate the starter solenoid and get you going. Trace the wire from the battery to the wiring compartment, turn the key each time you come to a connection, and find the spot where the volts first drop. Replace whatever is in the wiring between the last place the volts do not drop on your meter, and the first place that they do.
p.s. You can check your battery ground by putting the (-) of your meter on the battery (-) post, and put the (+) on a bare spot on the frame. Then try the starter. If the frame jumps up to a +voltage, then the ground strap is not making a good connection. It might show a little bit of voltage rise with a sensitive meter, but it it is half a volt or more, improve your ground.
(On a positive-ground vehicle, reverse the meter leads, but look for the same voltage rise across the ground strap.)