The xtrail is 134 bhp, not 136.
The brochure will show you 136 hp, note that's hp not bhp; the hp value is equivalent to ps.
This is currently a devious method used by manufactures to mislead the less knowledgeable.
A quick look at what cars data will confirm my point.
So the xtrail is even less powerful than you thought.
Fear not. I tow with the 134 bhp dci. It will easily tow your caravan. I presently tow a 1330 kg bailey. Also the 100kg nose weigh available is useful, showing no visible drop to the cars rear when hitched up, using 85kg
Mostly sixth gear is usable giving around 60mph for around 2200rpm. 231 pound feet of torque makes the X a great tow car. Look for around 30mpg towing.
Only on the long steep motorway inclines will you find yourself dropping down the gears. I can't say for mountainous area towing because I don't do any in these regions.
I have no wallowing while towing, or solo, strange I have read two posts that mention wallowing? My previous Audi Quattro had lowered sports suspension; so stiff I avoided potholes given the chance to avoid the jarring ride.
Check the engine front for signs of oil leakage. A common fault on 2004 models onwards, is a leaking air intercooler.