Why not change it at ~3000 miles while on the road? 15 min at a quickie lube, while your tailer is back at the campground. If this is hopelessly inconvenient then I would not worry about it. A few extra miles on an oil change when on a trip will not matter.
I do take a few minutes to pre-flight my tow vehicle before taking a trip though - check all fluids, check cold tire pressures (including the trailer tires), tighten lug nuts, and so on. Might be a good time to change the air filter too.