Max Practical Length of 5er

34' is enough for me. Sometimes it's the wheelbase of the truck that's a problem too. The longer that is the more careful you need to be when backing, especially in narrow roads. You can find yourself running into the ditch and taking out bushes if you're not careful.

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As for streets, the only issue really is height. Most full profile 5er's now are 13'6" tall, so low bridges, trees, etc can be an issue. a 40' 5ver wont get into all the same campgrounds as a 30'. You have to determine what type of camping you prefer, and the places that you would go. Many of the state parks have a reservation system that will block out sites, based on length, so you can get an idea of site availability by length.

We went from a 30' TT to a 40' 5th, and have no issues with any place that we normally camp.

the only other issue to think about is fueling when on the road. You don't want to pull into a tight gas station, or low overhang. When we are on extended trips, I try to use the busier truck stops, and either use the truck lanes, or in the case of pilot/flying J, many have special RV lanes.

I think the maneuverability question depends on the skill and confidence level of the driver. And yes, longer lengths (above 35' or so) will restrict your ability to get into some campgrounds, especially state and other non-commercial parks.

My recommendation is to go with the shortest length you can find that fits your needs - that's my definition of "practical".