First, you might consider selling your 5th wheel yourself rather than trading it in. You'll come out ahead.
During our full-time years, we went from a travel trailer to a 5th wheel for 8 years and our present motorhome for the past 10 years. The 39' motorhome you're looking at will have plenty of space for you. We had no problem with the change. It will just be a different way and places to store things but you'll figure it out. You'll also have a huge basement which is nice.
The reason we switched from a 5th wheel to motorhome was mainly because we didn't like driving the big truck around all the time. Also, friends latched us on to 4-wheeling and we loved it. So we got the motorhome and Jeep Liberty and they have both worked out very well. We have no regrets.
As far as your 39' size, we fit our 40' in public parks with ease, as they are always our first choice. We've used national parks, national forests, state parks, COE, county and city parks, along with lots of good boondocking in the West. Our front windshield has faced some glorious scenes of lakes, rivers and mountains. Of course, we stay mainly in the West.
Driving the motorhome is a real pleasure. It's smooth riding and easy to set up. Fun travels whichever way you go!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel