350 watt is fairly easy install next to the power distribution and converter area. Use the main 12v power feed to get 12 volts from the battery and use a small transfer switch to power the branch circuit of outlets. 350W should power the absorption fridge heater.
1500/2000 watts really needs more battery and needs short heavy wire direct to the battery. Then you pull 120v wire to the power center and again use a transfer switch to power the branch circuit of choice such as the microwave. Microwave far prefers a sine wave inverter.
And yes my RV is wired like this.