The 23 double axle is the most mobile. You will later go to a 25 foot. Skip the 23, to the 25 foot models. Keep it for as long as you are willing to use it.
The 25 double axle is just as mobile and a lifetime keeper.
The 27 foot / 28 foot is just as mobile as the 25 foot once you have several years of understanding what you see ahead and visualize where your REAR BUMPER will clear or not. The 27FBQ axles are excellent. Adding the 23 inch lift and 16 inch Michelins... never had any problem. You will encounter a road that you can easily determine is not worth the risk over time.
For those really anxious to get back further, leveling blocks and two 2"x10"x4foot long pine board is also an option. Use the excess piece to put onto the ground for the leveling blocks to sit, when detaching.
You only need to get into the area to explore. Detach the trailer and drive ahead to get a look at the road and possible better campsites.
After you grain experience, your education and skill sets improve. We have walked some roads, parking the trailer, to estimate the difficulty or ease of getting to a potential campsite.
Airstreams are very adaptable. The construction and cabinet hinges are poor compared to an Oliver Elite travel trailer. Once everything within an Airstream is upgraded, secured and reinforced interior... it can keep up with an Oliver. Otherwise you are limiting yourself to RV Parks, parking lots and more crowded conditions. We make our opportunities.
A 30 foot Airstream can travel well on a Forest Service road, straddle ruts, if needed. The risk is the grade angle that may drag the rear bumper. With someone watching the rear bumper for, drag... and whenever I am not sure... I recheck what Nancy sees.
Teamwork and Experience. There are some trailer owners that would be advised to RV Parks. It is just not their 'thing'. We are not risk takers. We know what we are doing from years of experience. Calculated Risk is learned.