2003 chevy 4 x 4 silverado 100 HD overheating when pulling our 5th wheel
 

My truck is a 2003 only has 42000 miles on it and we have pulled 5th wheel trailers in the past but past month it's been over heating when we r pulling. Fine when just driving without 5th wheel. Anyone experience this before. We r pulling a 2001 komfort 5th wheel 27ft. With A Small Slide out. Approx weight us 7400. We changed all the fluids before our trip and changed out the thermostat and pressure washed the whole engine. Any suggestions on what could be wrong?