Peter
There is no known parts list existing for the Magnum chassis as they seem to have been destroyed or deleted over the years and many ownership transfers of the Beaver coach corp.
How ever many of us here at the BAC have been through and sourced the parts your looking for.
Your Magnum chassis front suspension is a four link setup with two track bars these are known as torque rods unfortunately these particular torque rods are a spherical ball type units and as such the end balls are not serviceable meaning the rods must be replaced if they were out ,they where originally made by Dayton for use on Fruehauf semi trailers .
These torque where made in several different lengths for different applications but as the demand for them dropped the current manufacturer only provides a weld together kit to allow you to create the length you need .
On your coach that length should be 40 inches center to center.
If your ball ends are loose the rods will have to be replaced but if the ends aren't loose you can sometimes free up the end drill and install grease fittings and new seals which have found to fit the rods this procedure is should in the coach assist section under common problems .
If the mounting bracket broke off the frame it is likely that the ball end had seize on the torque rod causing the bracket to twist back and forth with the suspension movement until the bracket failed.
Here is a link to the rod/s you need
https://www.truckspring.com/products/Torque-Rod-Fruehauf-3-Piece-30-to-40-Inches__TS-345-812.aspxThe harsh ride can be helped by using Koni FDS shocks which are valved differently than the stock Bilsteen shock.
It's also important to have your coach weight for each corner and set the tire pressures to match the weight they are carrying in accordance to the tire manufacturers chart.
If you are experiencing porposing on bumps the installation of motion control units sold by super steer will help eliminate that.
Wandering can also be addressed by checking all steering linkage for play and checking ride height against the manufacturer spec as too high a ride height will result in wandering and the feeling that your riding on a bottle ( sliding off one side or the other)
Of course there is also alignment issues which can cause poor handling and don't forget the rear end the coach can have a rear steer issue caused by loose or worn torque rods back there as well.the torque rods for the rear end are much shorter and use a different sized bolt mounting the part number for those kits is 345-802 as below.
https://www.stengelbros.net/345-802-Uni-Rod-Kit_p_11489.html?gad_source=1&gclid=CjwKCAjw8diwBhAbEiwA7i_sJZO_-JJ91Uz_bCjH1Gy8oR_gzHcQO-ooEKnNDrDbYArwq_JwFL8TiRoC8aQQAvD_BwEHope this helps
Eric
Recent Posts
Locating parts on a Saturday afternoon wasn’t going to happen so settled for welding the bracket back on and dealing with the poor handling till we get home. 
Thanks! Pete