Looking for a certain type of car?

A pop up camper is light weight and has little tongue weight and can be towed with a smaller vehicle such as a crossover. However, unless you need a new car buying a car just for the MPG is foolish and will not work. The sales tax and registration fees alone shall be a huge factor. You obviously do not care for fuel efficiency for you have this truck since whenever and that 9 MPG did not bother you then nor since until now. The problem with buying a "suitable" car with higher MPG is that they do have a limited maximum total weight and limited towing capacity. Add the two and the vehicle shall be maxed out in weight and tow and will use a lot more gasoline than expected.

Prius isn't what you need.

Towing capacity sucks, and their open road economy isn't that great either. They get good economy around town because of the hybrid drive, where they can shut off the engine in slow stop go traffic. But sitting at 60 mph on a trip, the gas engine has to work the same as any other car engine.

A Toyota Matrix has a decent 1500 lb tow rating. With the larger engine and AWD options it might fit the bill better?

There are small pickups, the Mazda's and Ford rangers, with four cylinder engines and five speed standard transmissions which will consistently get 25 MPG. Also some of the toyota's. You won't find new ones though as they all quit making them. Also they won't get 25 MPG while towing a popup. Anytime you tow you're going to decrease your normal mileage. The Rangers with a V6 will get around 20 but when towing it will drop to around 15.

you will have to get a body on frame sedan to tow a camper. something like a buick roadmaster or a lincoln town car. they have V8's. these can tow more than normal cars. most normal cars are only rated for 1000 lbs. both get in the low 20's while towing if you are easy on the pedal.

Get something with high highway mpg if you are really going to drive all the way. There are small trucks that hit 20-30 mpg, but that's without towing anything.