I have the same problem with 2004 marquis 42. Increased the tag air bags to 60 psi from 40 that increased weight on the tag axle and shifted about 900 lbs to the front axle and lightened the dual axle with little difference to porpoising, also the higher pressure in my tags limited the lift height and increased time to lift the tag axle.
Next, I reduced tag air pressure to 40 psi installed Koni adjustable shocks on the front axle and set to minimum settings on shocks with little difference from Bilsteins. I then increased rebound / compression to 1/2 max and porpoising was reduced but small bumps and pot holes were much more noticeable. So if your goal is to control porpoising, Koni Adjustables do work but you may not like the ride.
I'm not sure if I will stay with the Konis, apparently they don't make a FSD koni shock for the magnum 10 air bag chassis. The FSD's might be the best solution if what they say is true that they soften minor road bumps and still control porpoising.
Koni does make FSD's for 10 air bag roadmaster.