The axles on our KZ TT are rated 4400 lbs as per sticker on axles and are 3" dia. Springs have 4 leaves. Also have Dexter EZ-Flex equalizers. The underside of our frame sits 24" above the ground. The 4th leaf is pretty short and can't do a whole lot. If your KZ 324BHS frame sits much less than 24" above the ground, that would suggest the springs need replacing. I can't imagine there is much of a weight difference between 3 and 4 leaf spring packs and sounds like more of an excuse.
It sounds like you need new springs. I would check the bushings. If the original plastic ones, they could very well be worn out. Ours were shot in just the 2K mile trip from Indiana to the west coast. The shackles could also be worn and ovalized. If the axles are 3500 lb, they are probably 2 3/8" dia. You might want to check the axle tubes to see that there is a bit of upward camber on them. Sometimes when axles are loaded to near capacity they can lose their camber from hitting bumps and potholes. I think what I would do is replace the axles with new ones along with new springs, rated 4,000 lbs or better, 4400 lbs. Not terribly expensive. I'd also suggest LRD tires. We have factory optional Marathon LRD.
Don't know what the correct axle/spring rating is for that particular model and year. Google is coming up with different numbers and as low as 7000 lbs. I've heard that in around that era, KZ was throwing in whatever parts they had on hand to save $$. Alko is now part of Dexter and you can't contact Alko anymore. I replaced the bearings last fall with Timken. I had a lot of trouble getting the correct part numbers, and someone (can't recall who) said that our axles are really 6,000 lb with a 4400 lb label and that there is no such thing as a 4400 lb axle/spring assembly, but the Alko website says otherwise.
My experience with 2 KZ Spree TTs is that the NCC is a lot less than what the factory brochure says. In our case, just one full holding tank would have put us over the GVWR. I did a detailed weighing of everything added on top of the UVW for mods/upgrades, camping gear, food, clothing, etc. and came to only 674 lbs. Just doesn't jive with the factory UVW. Have you weighed your 324BHS? Maybe you've unknowingly overloaded it?
If it helps, here is some Alko axle info.
1900-3500 lb axles and
3500-4400 lb axles The 3500 lb axles were available with an optional 3" axle tube and with 3 or 4 leaf springs. The larger axles were available rated 4000 lbs with 3 or 4 leaf springs. A bit confusing...
If you do upgrade the axle rating, you probably will need to replace the wheels (to 6 lug) and brake assemblies (larger dia. plus 6-lug).
The frames on the Spree trailers is 3 pieces of steel welded together and look like a regular one-piece I-beam. These frames are weak and flexible. I would check the frame above the spring hangers for cracks. If your Spree has a lot of miles on it and you've hit a lot of bumps, the frame can flex a lot leading to the cracks. If your springs have flattened for the same reason plus has inadvertently been overloaded, that is more reason to check for cracks.
