[JRTJH]

If I were you, I'd put that carpenter's level (6 footer) against the tires on each side of the RV. Measure the distance from the wheel lip on the forward wheel to the wheel lip on the aft wheel. Then measure on the other side of the trailer. If the axles are "parallel" that measurement would be the same. If there's any "deviation" from parallel, the measurements will differ. Assuming both axles are parallel to each other, you can also "expect" that they are PROBABLY parallel to the trailer frame rails (meaning the axle alignment is OK).

When I do that measurement, I use a adjustable square and slide it along the carpenter's level until it contacts "metal on the wheel lip" and mark that spot, then do the same on the aft wheel. The distance between those two "metal contact points" (measured on the carpenter's level) should be equal on both sides of the trailer.

Here's a picture of the type square I use: