Multiple Feed Power and Distortion Challenges

I have a system of electronic equipment mounted in a vehicles. I have a temporary arrangement of portable single phase generators (typical house generator) using multiple outputs (120) to feed into a power distrubution box which is then switched and distributed to vehicle electronics and ground support (cooling systems) equipment.

It is a bit of a kluge to demonstrate/test several vehicles at the same time. I have power cables running amuck since I have several systems being tested simultaneously that are physically located closer to support equipment (normal fielded application is 50 to 100 feet away). Thus I have quite a ground cover of coiled power cords.

Under the circumstances of electronic equipment (switching power supplies), I do find, and somewhat expect, harmonic distortion as seen by a power quality analyzer. I feed this distributed power to some UPSs (yet another switch mode power supply) and measure total harmonic distortion (THD) as high as 29% (3rd harmonic surprisingly high). In one case a UPS goes into "disable bypass" mode indicative of incoming frequency and/or voltage being out of tolerance (theory is harmonic is high enough in magnitude to cause the UPS frequency counter to read 180Hz?).

Is 29% a reasonable THD? Is it reasonable to expect that such distortion would cause a UPS bypass sense to trip? Should I consider filtering/conditioning? Should I use a more capable generator(s)?