I also tow a camper trailer regularly with my 3.0i/5MT X5. I weighed the trailer last year on a trip home, and it ends up around 4,000-4,500lbs, with a gross vehicle weight of over 9,000lbs fully loaded. So my X5 with trailer is probably slightly lighter than your X5 with trailer, but has a lot more aerodynamic drag on the highway, being a short but full-width and full-height trailer.

Of course with the 3.0 it takes a lot of effort to maintain speed while climbing hills, but I am able to maintain speed on the highway without downshifting. Especially at sea level I don't have any trouble on the highway in 5th gear unless I am trying to go 75mph or there is a serious headwind. (I live at 4,000ft and there's a noticeable difference at my elevation, but still it is possible.) Granted I do not own the automatic trans, but based on this experience I would assume something is wrong with your transmission if it is overheating just trying to maintain speed on the highway with your trailer.

(On a further note: I climb a mountain pass with my fully loaded trailer at least once every year, going up to about 5,500 feet on steep grades in the middle of the summer. These trips always have multiple climbs where I am spending 10 or more minutes at WOT, usually switching between third and fourth gear, at 95+ degrees outside, and I have never had a problem. I'd challenge anybody to think up a scenario that's tougher on a motor/transmission combination than spending ten straight minutes at WOT in between 4,500-6,000 rpm!)

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2003 3.0 5MT Topasblau
Purchased in 2016 and sold in 2024
2012 35d Platingrau