Boogie_4wheel is giving you good step by step instructions. I will add one more piece of information. When you first turn the key on, the fuel pump will spin, and the injectors will pulse(spray) a small amount of fuel into the engine. You should be able to hear the electronic fuel pump spin, as well as hear some rapid clicks from the injectors. Verify that you hear these clicks, as well as see the corresponding fuel spray. If you here the clicks, but do not see fuel, then work backwards up the fuel delivery system. If you do not hear the clicks, then look at the electrical. Check fuses that feed power to the ECM and power to the fuel pump relay. Also with that old of a coach, check the ignition key switch, make sure it is actually turning on.
BTW: When you first turn the key on, the ECM does its pulse thing, however it will not repeat that function unless the ignition is turned off for about 30 seconds. Also, when the engine first turns over, the fuel injection is off for the first few seconds of cranking, because it is assumed that the initial pulse is enough fuel to start it.
I would suggest that it would be very unlikely for your ECM to go from fully working to completely dead. Normally when the ECM fails, it gets flakey and intermittent, rather than just outright dieing. The ECM has redundant back systems to keep the engine running, even when it fails. If it really was bad, you should be able to get an ECM from a junk yard for about 35 bucks or tested one from Ebay for about 50. But it is very unlikely for the ECM to be bad. Much more likely to have something mechanically fail, like a switch, or some electrical connection.
