Suggestions for older pickup to tow 18k lbs 40'+ FW

Unfortunately as others have pointed out a truck that is capable of the trailer you are looking for will have to be a diesel dually and most likely out of the price range you mentioned.

Out of curiosity why do you feel a truck/fifth wheel combo would be more family friendly than the motor home with a toad? Seems like the comfort of a motor home while going down the road would be more family friendly than being crammed in a truck for long periods of time.