From symptoms you describe, oil has been recently changed, and is leaking. In 54 years of driving I've had dealer mechanics (call them technicians now) at least five times fail to properly install an oil filter or drain plug.
Improper installation of a spin-on oil filter, engine might pump out all the oil when first started, or it may take some time for the pressure to blow out the filter gasket, but in either case the oil loss tends to be sudden and massive. For one of these changes, our 1960 Country Squire dumped all of its oil within two blocks of the Ford dealer.
For problems with the drain plug (not tightened, over tightened, bad O-ring or gasket not replaced) the oil will drip out rather than blow out. It will come out whether the engine is running or not, but will come out a lot faster when warm. New oil is usually golden in color (some companies once used dyes for brand ID, not so much now). The oil doesn't go black until it is pretty well used.
Worst case for oil loss is often failure of an oil seal, front or back of the engine. This would be unlikely for an engine with 15,000 miles. But it is something I would have a mechanic check, i.e. from where is the oil leaking.
I check oil level before starting a trip, and always after a change. On a newly acquired vehicle I will check the level every time I stop for gas, until I understand how the vehicle uses oil. Most today simply do not, will not use a quart in 3000-5000 miles. In the good old days they might use a quart in 500-2000 miles, that's when we checked fluids every time we got gas.
Do you know how to check tire pressures? That's something else that should be checked before each trip in a RV. We used to check tires at gas stops too, but tires have gotten much better, though not as much as engines have gotten better. Tire pressure monitoring has become standard (mandatory) on some classes of vehicles, but not yet on the heavy duty trucks used for motorhome platforms.
