FWIW, I too wanted to upgrade the torflex axle that was undersized for our Casita but, not wanting to do a cabin off frame build. I considered or wished for the above mentioned trailing arm and Timbren but both would require either a complete new frame or sub-frame. The solution we went with was to remove the torflex, add steel to strengthen the existing C channel frame, extend the tongue, leaf springs and the main ingredients were bigger bearings and brakes that now ride on 265 x 16 E Coopers at 40 psi. With a WD hitch it does not sway at all behind the truck and has been very stable.
I'm not saying our route fits all of course, it just solved our overweight problem and so far the cabinets have remained closed underway.
The below was posted on another forum by Larry Gamble who has converted many heavy Casitas to straight axles as he describes.
We originally started doing leaf conversions not for lift but for chubby Casita's The #11 Dexter torsion axle is quite expensive and OMW shocks won't fit. For about the same price one can get a 5200# Dexter leaf axle, 4200lb springs, giant bearings, 12" brakes and Dexter shocks. But there is really no lift involved. The leaf axle actually moves the axle away from the bottom of the body and puts it in the center of the wheel reducing clearance. The project requires careful measuring, lots of cutting and welding.
We usually (on heavy 17's) use a 5200 lb axle but use 4200 lb springs. There have been exceptions where we used a 3500 lb axle and springs but these trailers were light. The motivation here was folks who towed in the mountains almost exclusively and perhaps had tow vehicles on the small side for a 17. 12" brakes are standard on the 5200. It should be noted that the 5200 axle is rated as such only because we order them with 6 lug drums. If we ordered them with 8 lug drums the rating would be 6000lbs. The key is lighter springs so we don't create a harsh ride.
We use off the shelf trailer springs. From a Dexter distributor we use their PR4's for 4200lbs or PR3's for 3500. They measure about 27" inches. With tall perches and long shackles this gives us a bit more then 3" of travel before frame interference.
