So, I brought my Fluke home from work and did some testing...
I plugged in the trailer with battery disconnected, there is a rock steady 13.5 volts at the battery wire. I replace the fuse holder and checked the battery, there is only 10.4 volts (probably a shorted cell).
I installed the fuse and re-checked voltage, there is only 12.5 at the battery but inside at the 12 volts fuse block there is still 13.5.
Looks like I have a bad battery.
1998 Jayco Eagle 26 foot 5th wheel (30th Anniversary Edition)
2003 Chevy Silverado 2500HD Duramax Diesel, Allison 5-speed auto, crew cab short box (460,000km and going strong)