There is a very long thread on this forum that responses to your question--in fact, Dave's earlier input to your thread was pointing you in that direction. Not to cover old ground, but here goes--the solenoid on the back of the battery bulkhead is the secondary to the primary on the starter itself. The starter and primary solenoid are hot-wired to the battery bank via hi-amp cables. The secondary solenoid is tied to the ignition switch via low-amp wire [eg 10ga]. You can test whether the ignition switch is providing power to the secondary solenoid by testing the smaller ternminal[s] for voltage once the switch is placed in the start position. You can also see if the primary solenoid and starter will function by jumping the two large terminals on the secondary. Again, there is a substantial thread on this forum about the secondary failing [either open or closed]. If open, it wont start; if closed, the starter can run away and over heat....keep us posted.....
2003 40' MDTS
Garden Ridge, Texas