My guess is the older MH had better power to weight ratio so it was set to hold in the higher gear on steeper hills. The new one presumably is lugging the engine on less steep hills, so it downshifts.
There's a reason it wants to downshift. If it does it once in a great while, just let it. If it's doing it every few minutes, turn the cruise control off so you can bleed off a little speed rather than downshifting or manually drop down and run in a lower gear.
I find if there are any significant hills, it's usually easier to manually run the throttle as I can feather it to bleed off some speed on the uphill and pick up speed on the downhill without a lot of downshifting.
You don't give the age but does it have the tow/haul mode that a lot of the newer pickups have? They seem to do a better job.