1998 boxer motorhome?

my boxer has picked up a nasty rumbling noise at between 50 and 60 mph. as the m/h tends to sway when moving. when level sound goes but when the camber of road places weight on either side its back which is most of time. i have both rear w/ bearings checked, alko rear chassis greased, 2 new front wheel bearings changed and rotated the tyres. still there. i have been told there is an intermediate bearing on near side drive shaft that could cause this. anyone an idea please