Just read a post on another forum describing a 20 amp dpdt switch install to bypass the inverter.
Here is one on Amazon:
http://www.amazon.com/Heavy-Duty-Toggle-Switch-Center/dp/B001FRE1E8
http://www.amazon.com/gp/product/B00004WLKA/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687602&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B001FRE1E8&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0BWH1W9D9B0QJ5XMWAE2
Wire the output of the switch (the "center" terminals) to the load, one input terminal set to the 120v line source, and the other input to the inverter output line. When you put the switch to the Line side, you have a direct path from 120v line to the load (wall outlet or whatever you are powering), thus bypassing the inverter altogether. When you flip it to the inverter input side, the line power is removed from the load and the load receives power from the inverter line instead. It doesn't matter if the 120v line is hardwired to the inverter and hot, or if the inverter remains on, cause the inverter output line is terminated at the DPDT switch unless it is in the inverter-feed position.