My customer has an older Coleman 5Kw Powermate PM0525202.
Problem is that the receptacle is different - it's got a 6-20R rather than an L14-20R (one flat and one 90deg rotated blade vs. 4 prong twistlock). I checked the Coleman site - no schematic. I assume the neutral and the GND are bonded inside the unit. There is a separate 120 v duplex on the unit.
Has anyone ever changed out the receptacle so the neutral and GND are brought out separately. I plan to back-feed the panel with a Square D interlock.
You could meter it out. If the neutral is bonded you should see 0 ohms between the neutral and ground on the NEMA 5-15s and the ground on the 6-20. To be connected properly your transfer equipment should also be switching the neutral.