Poor gas milage

Thinking you are getting poor gas milage,Your engine is running smooth,no misses,Engine runs smooth while accelerating up a hill, Still poor gas milage, Check your tire pressure. I have a 1999 Chevy Blazer 4x4, 4.3 Ltr. Auto, loaded. I have just over 154000 miles on it,I change my fluids every 5000 miles. Summer driving milage got real bad. I drive a Blazer hard. I tow a 16ft trailor usally loaded to the top with wood. Tires get hot in the summer, while in the winter, they stay mostly cool. Always check your air pressure.Most vehicals are equipted with radials now days, So if you think a tire looks low, check it, Usally they low. Usally in the winter months, nobody checks tire pressure. It really does make a difference. I went from 17.2 MPG to 19.5 MPG, while pulling the same trailor loaded with wood. I cut my own wood then stack it in my trailor to sell. I have Good Year Wranglers size: P235/75R/15 I just replace these with new ones. The old ones are going on my trailor. Counted milage on tires are 65000 Miles. Tires were rated at 40000 miles, Under normal use.