5th wheel style
WOW, M./M. : WELCOME, is your 1850 the Gooseneck 5th wheel style ??
Anyhow I have been doing quite a lot of battery checks last two weeks as I just acquired a 1986 F 1850 .. LIke said above the WHITE goes right to frame ground (NEG) if your sheet metal screw looks rusty /Thin. Drill a slight hole thru frame & file & put a nut & bolt tighter ground fastener.
The smaller in line fuse blue / black wire is the POS , Will feed the outside hitch light ,near the gooseneck. The thicker RED wire is also POS & feeds the 12 volts back into the converter (Location under the dinette cushion of the street side or drivers side. A screw can be opened to view the four glass fuses of the overhead lights, etc.. That same thick RED wire brings up the converter charge from shore or house 110volt power. Can check this value with a meter if off battery as mine is only suppling NINE volts instead about 13 volts. .... 2) There is the two RV breaker levers inside the wood door right near the FLOOR , under the Refrigerator location.. One is 15 AMP for wall outlets & the converter. The 20 amp will be Air cond circuit. .. Let us know your progress.