Properly designed rv wiring usually shares the microwave circuit and the 120v water heater circuit using a load shed box.
Having a extra breaker slot may not mean that you can use it. National Electric Code 70-551.42 requires that if you exceed five circuit breakers (excluding the main) you have to use a electrical management system. This could be a load shed box that allows the water heater to operate, but if the microwave draws more than a minute amount of current, the water heater is shut off. Then, after the microwave has stopped for 30 secs the water heater is switched back on. My Winnebago View motorhome had this setup and it worked quite well.
A Parallax ALS-20 is an example of this type of unit.
http://www.bestconverter.com/ALS-20-Automatic-Load-Shed_p_198.html#.X1Vs6FVKiJAThe control panel for the water heater is an Atwood/Dometic 91230
https://www.amazon.com/gp/product/B009UPKO3E/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1THIS PANEL IS WHITE IN COLOR. I DO KNOW THAT DOMETIC MAKES THEM IN BLACK, BUT HAVE NEVER BEEN ABLE TO LOCATE A P/N FOR THEM IN BLACK, BEEN ALL THRU THE DOMETIC WEB SITE AND ELSEWHERE. IF SOMEONE CAN FIND THE P/N FOR THE SAME PANEL IN BLACK, PLEEZE... LET ME KNOW.The 120vac goes directly to the back of the water heater, and is controlled by a relay that is controlled by the 12 vdc. The four wires from the water heater go directly to the switch and then you take 12vdc both positive and negative to the switch. If you need to extend the four wires from the water heater, use properly colored 16 gauge primary wire (Home Depot or Lowes has small spools of primary wire in different colors) spliced to the existing wires and best to sheath with the split wire loom material (found at auto parts stores or Harbor Freight). Stagger the splices and you don't get a huge lump in the harness. Best would be to solder the wires and heat shrink the splices. I used permanent Raychem butt splices with heat shrink sleeves over them.
I've done this............. found otherwise undiscoverable wood rot from water leakage when I pulled the water heater. Wished I had never tried to change the water heater, the whole trailer would have been sold and gone. Now I am stuck with repairs, gotta get motivated.... make room in the shop for my Bigfoot.
Charles
