Any circuit absolutely should be fused as close to the source of power as possible, the fuse rating being determined mainly by the ampacity of the wire. Fuses (or equivalent protection devices like circuit breakers) are fire prevention safety devices.
Standard practice, as others have said, is to have a 12V distribution panel/fusebox for the house loads connected to the house battery and individual circuits as appropriate for various devices. There are many RV specific power centers available, containing both 120V and 12V panels and often a converter/charger (to run the 12V items when connected to 120V power and to charge the house battery under the same circumstances). If you'll be having a 120V system, one of these would probably be the simplest and handiest way to set things up. The Progressive Dynamics Mighty Mini power center might be a good choice.
It is a good idea to have a separate battery and electrical system for the house loads and leave the chassis electrical system largely unchanged. There are a couple of reasons for this--the most important being that you can't strand yourself by running the starting battery too low with house stuff. There are various isolator devices that allow at least one-way charging so that the house battery can be charged by the vehicle alternator while driving. Some of the more sophisticated setups also allow the chassis battery to be charged from AC power by the converter, but that's less of a necessity in a lot of cases.
