Have you weighed your rig? I weighed mine the other day and found that my axles are more than adequate. for the rig. You have 3 6000 lb axles for a total of 18000 lbs. Even though your rig is heavy you are, probably well within your axles ability. Go weigh them each axle and see what you get. BTW, the internet is full of used axles, they are cheap (inexpensive), even new ones are cheap. Go to dexter axles website and look at the 6000 v 7000 lb and you will be able to identify the difference. I have not done it but I bet that hte only difference is the hub and bearing.
Another alternative is to go to a spring and axle shop and tell them that you want to put larger axles and bearings on your rig, without removing them they will cut the weld and insert new axle stubs in the housing then you can either get new hubs or install different bearings and races.---just sugestion
The 6000 lb regular duty axle use 3 inch light wall and a 2 1/4 seal because the spindle diameter is larger to accommodate the larger ID of the light duty tube, the 7000 lb axle uses 3 inch medium wall so does the heavy duty 6k, spindle is also the same across the board. Soooo the wall thickness is an issue starting at the 6k light duty.
It appears to me that the differences are: 1) dexter 6000 lb light duty vs. 6000 lb heavy duty is the wall thickness and a 2 1/4 in seal. 2) The difference between the 6000 lb medium duty Vs the 7000 lb is is the brake assembly possibly only limited to the magnet.
With just this brief examination of the specs, I think one might convert a 6k heavy duty to a 7k by changing the brake assembly and the only difference I can see there is the 7k magnet kit. A 7k conversion from 6k heavy duty axle may only need at brake magnet or a 7k brake assembly
I would rely on service factors since the bearings are the same, perhaps I would install the brake assemblies for the 7K or the magnet. I would invest in the various aftermarket kits to improve my suspension and be done with it. I think that the only thing you can gain between the extreems are thicker wall on the axle housing/tube and a magnet.
Make sure you upgrade the leaf springs. Both of my rear ones snapped. The dealer did a dance and heartland did a dance. 6 to 8 weeks to repair. Went to a guy which my Chapter director suggested. for 1/2 the cost I got all 4 springs( american made) and increased the leaf spring capacity for 250 bucks. I totaled the old capacity up and it was under weight by 2500 lbs.