Usually has more to do with wind resistance and gravity than the TT's size & weight but sometimes rear end ratio and engine output can be a factor. For example:
Our old 2000 F150 with 4.6L V6 (non-turbo) & 3.55 gears towing a 5K lb TT got as low as 5.6 mpg on longer/steeper grades like Grants Pass in Or. and could only make 30-35 mph flat out in steepest sections. IIRC, not ever much better than about 10 mpg. Good mpg around town tho.
Our heavier F250 towing with 6.8L V10 & 4.10 gears pulling a heavier 7K lb TT gets above 8 mpg longer/steeper grades like up Stevens Pass in Wa. with plenty of power to spare and easily maintains 60+ mph. Typically in the 8-10 mpg range on average.
General rule of thumb - slow down. There will be a sweet spot for best mpg, maybe in 58-62 mph range.