On my 345, it takes a bit of forward movement for the bags to properly inflate.
It seems that the control valves require a bit of "on the road" time (a couple of body roll cycles) to "find their groove". If you make a setting change, drive it around the block and observe where the "bag height distance" stabilizes.
Another point, if you have replaced the tag axel, once you get the bags set to where you think they should be, you really need to record the weight supported by each of the four rear tire support areas (the right and left duals, and the right and left tags). If the rear bags are riding too high, too much weight is being carried on the main axel, and the tags are not carrying their fair share of the load. Conversly, if the bags are set too low, the tags will be carrying more weight than what they are rated for.
The only way to check the proper suspension setting (bag height) is to take the vehicle to a scale and record the weight that each of the four rear axel road contact points (tire groups) is actually supporting.
The following threads contain pertinent information.
More threads can be found by using the "Search" icon at the top of your screen and do a search on "Ride Height", "Rear Suspension", or "Bag Pressure".