Yes, sorry..your trailer is probably too long for Chisos.
I really think you'd like Davis Mountain State Park. It's at an elevation of 5050' whereas Chisos is 5400' EL. There would be more to do in the immediate area of Davis rather than Guadalupe Nat'l Park.
However, Dog Canyon campground in Guadalupe is at 6300' EL but is very remote. It's about 70 mi from Carlsbad, New Mexico and accessed via NM137. There are only 4 RV sites there and no dump station. Pine Spring campground in Guadalupe in Texas has about 20 sites and is basically a paved parking lot but it has scenic surroundings. It's at 5800' EL. No dump station there either.
Hope you can find some cool temps!