Camper ?

I have a small cargo trailer, I get 38 mpg city towing go over 45 mph and MPG goes way south. The worst I got towing was when I bought the trailer and towed it home from Ga I drove interstate speeds of 70 mph with a very strong head wind. Worse possible conditions to tow for economy I got 20 mpg for that. When I got home (I pushed it hard) My car had gone low on oil and I did get it home but it was wanting to shut down due to low oil. I figure push it the hardest the first day that way the easy towing of the future will be done in security knowing your limitations.
WHAT IS YOUR ECONOMY WITH YOUR LITTLE HARBOR FREIGHT HOME MADE CAMPER? IF IT IS LOW ENOUGH I WILL BUY OR BUILD ONE BUT ONLY IF THE ECONOMY IS WORTH THE TRIP. I can add a few things to my current trailer and use it to camp in no problem but if I am to take it on a long trip I will have to get a small camper like yours.
I did attend the TRAVELERS CAMP FEST 2016 I was one of 2 Prius there My gen 2 and Kent State Ohio Bobs gen 3. We both slept in our cars and ran the engine for climate control. I did not have to run each night but it does hit your economy a little running your engine but all in all it is still cheaper than a hotel. I would wake in the morning head to the Planet fitness for a shower and restock on supplies and return to the beach. My car was packed way too tight for comfort hence my interest in getting a camper trailer. I already have the generator just have to decide weather or not I feel the expense is worth it to get a camping unit.