The 76 Winnebago Minnie Winne brochure indicates that you have a pressurized fresh water system not a on demand pump.
This means you have a steel tank under one of the bunks or a bench seat. Find the tank and follow the pipes to the outside fill and vent ports. Normally there are access panels under the bench seats or bunks. A pressurized system uses a 12VDC air pump to pressurize the system rather than an on demand water pump. You have to vent the pressure off the system in order to fill the tank. Only information I have ever seen for the pressurized system is in the 74 and earlier Winnebago Service Manual (Section 60-40-01). The manual is available through the store at http://www.classicwinnebagos.com/forum/index.phpe
House parts reference information is available at:
For the Dodge chassis portion you need a MB300 chassis Service Manual.
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The are no GFCI circuits unless someone added them. You most likely have a bad connection somewhere. AC is routed through the bins and overhead. Check for a junction box in the rear passenger side closet.