what is the cause of volume fade on mikomi t.v. in my motorhome?

mikomi 15lcd250 set power supply is 12v d.c. via a 240v transformer/ rectifier. I have installed in motorhome with 12v d.c. supply direct from 12v socket thus eliminating transformer. set is connected to 12v d.c. sagem freeview box for digi. t.v. a second roadstar t.v. is installed in motorhome in exactly the same way (his & hers). Both sets work perfectly together but the sound on the mikomi fades in distinct steps as lights etc. are switched on one by one. roadstar sound remains constant. voltmeter readings at mikomi socket go from 13.5v in steps down to 12.4v (m/home connected to mains hookup). picture quality on both sets constant throughout.