Towing a trailer with a gas truck will average about 9 mpg and those smaller tanks require too many stops.
My 26 gallon tank needs refilling about every 3 1/2 hours when towing. That's more than enough time for me to go without stretching my legs, taking a bathroom break, etc. Saving a couple of 15 minute gas stops per day seems trivial compared to the other considerations, such as parking, turning radius, fitting in the garage, etc. The bottom line is, it all depends on what you want your truck to do. That's why they build so many different sizes. :B