Sounds like you put your cart before the horse ... or your Kangaroo before your tow vehicle...
Kidding aside - an Expedition should be more than capable. The towing capacity in the owners manual should be more than your Roo weighs loaded, but double check. The spec that is usually the one that max's out first is paylod of the tow vehicle. You will want to check the door sticker for the max gross vehicle weight (GVWR). That minus the empty curb weight is going to tell you the available payload.
You can compare that to the total weight of passengers, fuel, gear, and the weight of the trailer hitch and the tongue weight of the trailer.
To estimate (on the high side) your tongue weight of the loaded trailer, take 15% of the max gross trailer weight. Should be on a sticker on the front, street side wall of the trailer.
There are some detailed theads here if you search that give you all the ins and outs of calculating exactly how much a given vehicle is rated to tow and there are some handy towing calculators you can enter your specs into.
And - sales people are salespeople - they tell you whatever they need to in order to close a deal, so they will almost always say yeah you can tow that no problem - but it pays to do your homework and check the calculations for yourself.
Congrats on the Roo!