I have a LVL-1 electronic level and place the transmitter on the kitchen counter near the sink. That puts it almost centered over the axles, which is what they recommend. If you have a more open floorplan, putting it on the floor in the same area is advised. Lacking the LVL-1, using standard carpenters levels works, too. Once you find "level" for your rig, apply the stick-on bubbles to the front and side near your jack switch. You could also mount one of those larger bubble levels on the pin box, once you're sure it's level.
So long as any swinging doors don't fall open or closed by themselves, you're probably as level as you can be. Another indicator is to look at the water in the toilet bowl. Is is uniformly distributed around the bowl or does it look like it's off to one side? Same with water in the sinks.
Now that I have the Ground Control jack system, leveling is a much simpler chore. No piling of Leggo's or boards under the tires, pulling the rig back and forth. If the site is really cock-eyed, all I need do is put my cribbing pads under the low side jacks before dropping them. And it doesn't leak, get dementia, suck your wallet dry (as much), or go bump in the dark of night.