Tom will need to run the suggested voltage checks first, and if it's bad the replacement commonly used is Cole Hersee SPST 12v 85A Continuous Duty #24059BP or similar. However you should check the info on your current solenoid before going after one. It's important in this application that it is a "continuous duty" version. These solenoids are a common failure point, as a Search of this Forum will reveal. A more expensive but reliable electronic alternative is available (Cole Hersee solid state 48785), but most of us just carry a spare solenoid as they are easy to switch out. If you do replace one, make certain that you get the diode in the correct orientation, with the gray stripe on the left (toward the "Hot" side. Cut the Chassis Main switch in the battery bay before attempting the change-out to avoid inadvertent short circuiting.
All that said, for Tom, Eric's Transmission Control Module fuse check procedure and reference to a troubleshooting guide in Coach Assist should be given primary attention. Simply checking voltage at the batteries is not covering all the bases, but asking here was the next right move.
My experience with common unsealed wet cell deep cycle house batteries reinforces Eric's that their gassing under charge pressure will corrode proximal metal coach components. Though my original Interstates lasted 12 years, they ate up metal tie down buckles and melted plastic ones, and ate into the battery slide-out tray. AGM's resolve that issue, as would Lithiums. On our coach, major fuses and electronics are in a box immediately above the battery tray; luckily corrosion did not infect anything in that box. But if Tom's TCM leads run nearby to standard watered batteries, I can easily understand Eric's point.
https://www.amazon.com/Cole-Hersee-24059-BP-Insulated-Continuous/dp/B0064MX7US/ref=sr_1_1?ie=UTF8&qid=1428011897&sr=8-1&keywords=12v+continuous+duty+solenoidA note of caution here: there are some listed versions of the 24059 that say they are 65 amp, and you want the 85 amp. The 24059BP I ordered as a spare will show with an "24059-08-BP" at Amazon, but the package doesn't have the "-08" on it and it's 85A. Apparently, according to one chart online, the "-08" designation is for 65A versions.
https://www.littelfuse.com/products/dc-solenoids-and-relays/standard-high-current-relays/continuous-duty-spstJoel
