I've got a few more add-ons than you, but my combined trailering weight is usually right around 13,500 to 14,000. Most of the time it stays in OD - long hills I sometimes back off to 3rd if I feel it's lugging. Keep the converter locked up as much as possible - easily a 20*-30* temp swing if not. Once you get your guages and get used to them, you'll feel more comfortable since you can easily see exactly what's going on. Even with a stock ECM, a little more boost will help. I personally think 14,000 to 15,000 is about on the edge of "comfortable". My towing mileage is about 13.
1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor