You should be checking tires every trip. If they are lower than required then add air to bring them back to where you want them. The pressure will drop as the ambient temperature lowers. Also; tires with lower pressure can loose air through the bead when cornering.
I prefer to keep my tires within 5% of sidewall maximum. Harder tires tend to hold air better.
There is no normal for tires because of the multiple reasons for loosing air; small tire puncture (nails, etc.), sticking air valve, leaking bead, etc. The vary reason the manual and maybe a warning sheet inside the RV recommends check tire pressure, inspect tires, and lug bolts each trip.