My motorhome is hard to control at highway speeds.

First step is to load the MH as it would be for travel and get it weighed. Best is each corner separately but at least each axle separately. Set the tire pressures per the load/inflation tables for your tires. Over inflated tires on the front end reduce the tire/road contact area and side to side friction with the road surface making it easier for the front end to slide left/right in varying crosswinds and when trucks pass. Next step would be an alignment.