You don't ask for a reality check, but in all honesty, 1 mph is pretty slow. Though you could mix in a bit of sloth for good measure, at about 125 feet per day. Not much of an ambush predator, unless your beasty starts its ambush last week.
Really big scavengers can be highly-susceptible to extinction. So make sure that there's plenty of food in your world to go around. The Short-Faced Bear is an excellent example of this. They were the biggest, scariest scavenger running around and then the climate changed and bam they're extinct. This is why obligate scavengers are rare and most of them only supplement something like hunting. If they were mostly ambush predators and secondarily scavengers that'd be different.
They need to be able to either eat stuff other animals can't eat (like a vulture) or scare predators away from their prey (like a Short-Faced Bear). Now if I'm the second-scariest thing in this world, this doesn't really sound scary enough to get me away from my well-deserved meal.
Scavengers need to be able to access enough carrion to support themselves, so they need a very large range. Unless there's so many animals in #1 that this isn't an issue. If they don't move around much they either need a different food model or there needs to be dead bodies everywhere.