Ok.. This subject gets tricky....
Motor homes.... Gun can be in the rear of living quarters... While in motion must be unloaded.... Parked, setup not in travel mode... It's your home. Residence logic applies... The gun can't be loaded anywhere near the vehicle/cab unless your are active LE or have a ccw honored in the states your are traveling through.
Trailer... All day long... Separate objects... I'd probably get a warrant to search the trailer, but I'd need PC anyway.... If I have probable cause to search the truck, I don't need a warrant..
This topic can get really in depth...