Inverter draws power from your 12V setup. You don't mention your battery setup but regardless of if you have a permanent or portable inverter it will draw from the battery/batteries. A small inverter (<300W) is fine with a single battery, but any bigger and you'll want a multiple battery setup. The advantage of a permanently installed inverter is, assuming you have installed it properly by putting it as close to the batteries as feasible and used adequate wiring, you will be able to utilize the full extent of the inverter's capacity. This is because voltage drop due to wiring resistance has been minimized. When you use a portable inverter, you are usually plugging into a 12V outlet that has inadequate wiring to support the draw that the inverter may try to pull. So your inverter will shut off due to low voltage long before it actually reaches its design capacity.
We simply plug our shower power cord directly into an inverter outlet (permanent install). Your camper's converter must be shut off somehow if you do this, we do it by flipping the converter's dedicated AC breaker off on the panel. All AC outlets are live doing it this way. Water heater and fridge are flipped to gas so they don't try to run off of the inverter and suck battery.
