We had the same problem with a 5er of ours. In the hot weather, it's really, really bad.
I was told by a Camping World Service Manager that if a coach is left out in the hot weather, unused, for a long enough period of time, the "fumes" basically are drying out the adhesives in the cabinetry & wall paper (or whatever the stuff is). If left for long enough, this drying out of the adhesives will cause those things to de-laminate and they can come apart.
He advised to always have our unit plugged in to shore power & turn the A/C thermostat on 90'. Ever since then, no fumes/formaldehyde odors.