The US had several towed 8" and larger guns.
The 240 mm howitzer M1 was towed.
Even the frickin 280mm M65 atomic cannon was towed.
Here's an
http://en.wikipedia.org/wiki/List_of_howitzers
Btw I am an engineer and served in the US Army artillery -if that helps.
Your definition of “towed” and mine differ a bunch! The M65 wasn’t Towed in my book: it is clearly carried by its transport and then emplaced on the ground. We have one of those puppies at Aberdeen and I would be pleased to show it to you. As far as I know, 240mm howitzer was rarely used -Anzio, as I remember.
You are correct that we had towed 8 Inch howitzers in WWII. Very clearly a large, unwieldy weapon on today’s battlefields. Should I ever be privileged to command a division, I would opt for the SP version..