I'll take another crack at it tomorrow.
I may be interpreting the schematic wrong but what I see is the 30 amp fuse comes off the solenoid battery lug (12vdc) and feeds the main switch (key) at pin 30. Only when the key is turned will this 12vdc be distributed through, from pin 30 to ACC in first position, BOTH ACC and pin 19 (preheat circuit) in the second position and then to solenoid trigger pin 50 in the Start position.
The 12vdc is present at solenoid battery lug and pin 30 of the main switch "until" the key is turned past the ACC position. That's when the voltage tanks. This happens with and without the positive cable attached to the starter solenoid (rules out possible internal shorting the starter/solenoid).
My thought, though maybe incorrect, is that when the key is turned to the Start position the 12 vdc feeds the acc circuits, the glow plug preheat circuit and the trigger (pin 50). A short on the preheat circuit could drag down the battery. I know when you hold the key in the preheat position the glow plugs (pins actually) are supposed to get red hot (haven't checked or disconnected that circuit yet). There must be a timing circuit to drop those out as they're not needed once the engine starts.
I can troubleshoot a train consist (electric or diesel), automobiles, RV's etc but this little BX2200 has me wondering. I need to get a hold of the schematics for the BX2200 (it's from 2001). The one I posted I found here on another thread.
Thanks again all.