OK - I think I've got it.
The reason that you have to raise the front of the Ollie to drain the tanks is common to all Olivers that I know of. This is because of the basic design of the tanks - they are fairly wide and long - but - not very tall since they have to fit between the inner and outer shells of the Oliver. The good news is that this design helps lower the trailer's center of gravity and more evenly distributes the weight of the tank's contents from side to side and front to back. In addition, the location provides protection to the tanks from sources outside the outer shell of the Oliver and given that in colder temperatures the area between the shells is somewhat warmer as compared to outside. Unfortunately, the downside to this location and design is that in order to completely empty the tanks, the front of the trailer must be elevated such that the liquid contents flow more completely (and faster) towards the outlet which is basically at the rear of the camper.
Your 40% seems a bit on the high side but that could be a sensor issue (again, it really doesn't take all that much of a mis-placed sensor and/or the degree of level of the trailer to have a fairly dramatic effect on the reading).
I think that most of us develop an idea/sense of when and how long we can "go" before these tanks need to be emptied and or what our readings really mean. But, in virtually all cases if we want to really empty these tanks, the nose of the Oliver must come up.
Bill