Just a little issue with terms , solenoid and relay , .
There is one of each involved in your starter operation ; the solenoid is part of the starter , and when signaled by the relay , engages the starter drive into the ring gear and powers the starter armature to spin the drive .
The starter relay , takes low amperage power signal, from the ignition switch and if the relay is grounded by a signal from the transmission controller ; indicating trans in neutral ; the relay passes high amperage power to the solenoid on the starter.
Unfortunately over the years the name , solenoid, has been used to describe both .
If , when you turn the key , there is no noise from the starter , the relay is probably your problem ; I can tell you exactly where it is on a 99 Freightliner chassis , I've replaced mine twice; but for Spartan chassis , I'll have to say , phone Spartan with your serial number and ask for the location AND a picture of the part so you know what to look for.
Spartan Motor Home Support.
1-800-543-4277
www.spartanchassis.com