Seems like your OP has been answered pretty well. And it’s what I would have said too, 7.5 mpg for a 35-footer with a big block Chevy *and *towing is, as someone said, fairly decent. And I think your SO is mistaken about *ever *getting 15mpg. My 2004 Nissan Pathfinder SUV with a 3.5L V-6 (~215 cid) only gets about 15 city and maybe +20 highway (with a tailwind!) And that’s not towing & with only my 180lb self in it!
Don’t be too jealous of diesel pushers either. Yeah, diesels do get better mileage pulling heavy loads, but:
[li]They cost a lot more up front to buy.[/li][li]Diesel engines cost **significantly more **to be serviced or repaired.[/li][li]They may get better mpg, but remember diesel fuel is more expensive (like $1/gal. more than premium gasoline right now!).[/li][/list]
One other thing I wanted to ask and it has less to do with mpg than it does with wear & tear on your towed Jeep’s transmission: Is the Jeep Liberty you’re towing a 4x4? If so, does it have the old-style hand-lever transfer case shifter or the newer-style electronic switch on the dash to change from 2x4 to 4x4 mode? If it has the shift lever then it will have a neutral ‘N’ position. Make sure you *always *put the transfer case in ‘N’ when it’s being towed.
If it has a switch on the dash though, those never seem to have a ‘neutral’ setting. But if your Jeep’s got the switch on the dash and a manual tranny, than just putting the dash switch in 2x4 mode and leaving the tranny out of gear while towing will suffice. But if it’s got the dash switch and it’s an automatic, things are different:
[li]DO NOT use a tow bar. EVER! This will prematurely ruin an automatic transmission.[/li][li]Put the dash switch in 2x4 mode then back it onto a car dolly (i.e. so only the front tires are touching the road). Big problem: Towing ‘backwards’ like this is often illegal in a lot of areas. Plus it can be a little unsafe because the towed car’s engine weight isn’t over the hitch where it should be.[/li][li]Use a full car hauling trailer (all four wheels are off the ground).[/li][/ul]
Main point is, with an automatic you **must **avoid spinning any ‘drive’ wheels while towing.
Important: Make absolutely sure you put the transfer case **back **into anything other than ‘N’ (neutral) **before **you unhitch! Else it will roll away even with the transmission ‘in gear’ or ‘PARK’! (I seriously suspect this is the reason the newer electronic 4x4 systems eliminated the ‘neutral’ transfer case setting!)