2003 Dodge Truck Trans slips

Two Questions; 2003 RAM 2500 5.9L Diesel
1) While pulling my fifth wheel, (13,000 lbs.) On a flat and depending on ambient temps, I drive in the southwest. My truck runs at 195 degrees F. And at times climbs to 210 plus degrees. The stat opens at 195 F. And spec. Is full open at 207 F. I do pull over the mountains and through deserts. Should I consider a cooler thermostat? Or is this ok, I do not experience any other cooling or heating systems problems. Radiator flow, fan and fan clutch is good.
2) While pulling, I experience an intermittent slippage. I have a 4 speed auto. With overdrive, I do not use overdrive while pulling. While pulling it seems like a hard jerk at times while climbing steep inclines or hardheadwind. Started in the last year, fluid had been changed in last 6 months, still pink no stink. It used to be a smooth downshift but now the hard jerk in and out. Are there some torque convertors or transmissions out there that might solve my towing problem? Any help would be appreciated.