Single to Double Axle advantages...
I have not pulled a single axle AS, so cannot make a comparison. But, I can tell you why I picked a double and how it handles on my 23 footer.
On gravel, you probably will not notice a difference between the towing behavior of a double or single axle AS. You probably are not going over 45 mph in most cases. Cross winds would not be a factor, but wash board at that speed would be excessive and 15 mph on a wash board road is plenty. If the trailer is bouncing one way or the other... too fast at the speed you are going.
Crossing a wash out in a road or pulling off the main gravel onto a side pullout is an advantage with a double axle. When one axle is going down, the other is going up. Always keeping an eye on the plumbing "dangling" there to get torn off.
A flat tire, blow out, etc. while moving with a double axle is less of a danger, than losing a tire on a single axle. At least you can pull off the road easier with three tires than one...
A single axle trailer is lighter, so sure, it will be easier to tow. The 23 footer is the first double axle available, but has 14 inch wheels which is a disadvantage. The 25 footer has 15 inch wheels, as do all single axles. You can only have C rated tires on a 14 inch and can get a variety of D rated tires on 15 inch tires.
My double axle tows nicely at any speed and even through cross winds of Wyoming without a stabilizer. With a full fresh water tank or empty, it tows comfortably, even in a severe cross wind on I-80. With a full fresh water tank of 30 gallons and 6 gallons in the hot water tank, it rides even better, but does cut down on your miles per gallon. I will keep a minimum amount of fresh water in the tank and top it off at the destination's last gas stop.
No matter having a single or double axle, keep an eye on screws working their way out and falling onto the interior floor, closet or cabinets. The rougher the road, the more screws you need to "snug" up. I always have an electric screwdriver to snug up loose screws.
Wash board roads are even hard to travel NOT towing. A crawl, slow or faster it is a road to avoid the next time around. Uphill wash board is the toughest to keep momentum. Going down you can sit on low gear and keep from bouncing. In most cases, there are NO sweet spots!
After a hundred miles of less maintained county roads you will find that larger diameter screws are needed to secure some fixtures that are prone to shaking the screws and hardware loose. Even securing panels to others with extra hardware to secure vibrating interior cabinets to one another if something you will eventually find as s good idea.
Overall, my 23 foot traveling all kinds of back roads, I can only say I am content with my AS. I might complain about how it is built for highways, it is not difficult to modify the factory hardware into an Off Road War Wagon! The AS is not designed for wash board and back road travel with ruts and uneven exits from the main road. Too low clearance. Most other trailers are built above the axles and AS has the axles built into a lower profile, making it nice to pull on asphalt and concrete highway travel, but you had better have eyes looking ahead for trouble to clear your plumbing, hitch bottom edge and rear bumper ahead to avoid tight clearances of trailer and road...
I would NEVER buy any other kind of AS, other than a multiple axle, IF you plan to do much gravel and hunter's camp pull out camping. If you are only asphalt, concrete and manicured gravel at a RV Park lot... go for anything that gets your attention!