Well... hope a new day has you in an experimenting mood. Or Coach Net was called. ('AAA Premier RV' card holder here

)
RE: "" I remember that it is a switch that you press once and it should trickle charge the battery from the coach batteries, this is very clear in the PC owner's manual. ""
Well.. if that is the case, then it should be so. To verify, IF you are on shore power, then the coach batteries should have a good state of charge and can charge the start battery. If not on shore power, maybe start the generator first.

Next turn on the headlights. With a dead/low start battery they should be weak. Then pressing the momentary switch, the additional house battery power should cause the headlights to brighten up, indicating that the batteries have connected, and are supplying current to the start battery.
If the lights dim again when you release the switch, that would indicate that it is a 'boost' and not a 'stay on and charge' function.
After determining what the momentary switch does, turn off the headlights and proceed accordingly.
Hope this helped!