Max Trailer Wt. Recommendations for F-150 5L or Eco/Boost

I had sticker shock when looking at new trucks. I'm retired so don't make the big bucks. I looked for used. I found a certified F-150 V8 2010, 6.5 bed, qd cab for $15,000. It gets about 12 pulling the trailer and just under 20 on flat freeway without the trailer. I bought a brand new Nomad 23 ft, 3600 lb. There is just the two of us. We love how nice the truck tows. We are taking trips longer now and each time we talk about going even further next time.