The LZ0 is the best of both worlds. Good towing and good mileage.
I tow a similar trailer with no issues with my 1500 duramax. I traded in the 2500 duramax for the 1500 and little difference in the towing experience.
Most of my driving is 25-45 miles one way so fuel economy is important to me.
However, the big caution is that owning a RV means changing the RV for the "next" one.
This means a bigger trailer after a couple of years.
So, I would caution against getting rid of the 3500. The 3500 is future proofing your RV experience.
I feel safe with the 1500 and not going to a bigger trailer. This is my last RV.
I do miss the 2500 from time to time when I need the larger capacity to tow a skid steer or something once in a while. But not enough to matter.