Your Itasca Windcruiser is the Itasca version of the Winnebago Elandan with a few styling changes. You can contact Winnebago Industries and they will send you wiring diagrams, build sheets, etc. for you motor home. You can get the 1990-on wiring diagrams here: Wiring Diagrams
What you are describing does not sound right. The first thing I would check is the power supply underneath the settee to make sure all of the fuses get power when operating on house batteries and that the house batteries are delivering full voltage. Also check if your electrical appliances work on shore power.
The dual battery switch controls the battery connection through two solenoids on the front left firewall. They might be acting up which can cause quirky electrical behavior.
I don't think this will solve your problem, but hopefully it will get you going in the right direction.