Tandem tows better, less prone to sway, better recovery from a flat tire, BUT after years of towing tandems my current camper is a Gulfstream Vista Crusier 19ERD which is 24 feet long and a single axle, GVWR 4100 camper set up correctly with blue OX sway pro and it tows great. I looked at a lot of campers when you start getting light weight, and get below 4500 pounds tandems are a bit more rare. The second axle adds a lot of weight. I have towed ten hour days with my camper and have used it pretty much last year and have no regrets.......Jack
Good plan on the 4,000 , Gladiator tows the camper great and we have been up some good hills, Weight of my camper after A/C and options added was 3100 pounds, and we try and travel with low tank level in fresh water.....Jack
A lightweight single-axle trailer may be able to use LT tires instead of ST tires, because in a hard turn you are not twisting the tires in place. LT tires usually have a higher max speed, fewer blowouts (never had one!), and can easily be found Made in USA.
A single-axle trailer is sometimes available in an off road package with extra clearance.
An all-terrain tire tread can be a better traction match for the tow vehicle. You don't want the trailer to skid easily compared to the tow vehicle.
On the other hand, many people don't want to fit into a small trailer.
After having several campers and motor homes, look for a floor plan that best suits your needs then see what models from different manufacturers fit that need. After that, then see what their weights are. If all things being equal, I prefer a double axle and would take the couple of hundred pound weight penalty and take the extra axle.
In my experience, a single axle allows you to get in/out of tight camp spots...I guess it depends on where you like to camp. This camper will turn on a dime when backing up, which is a bit challenging but also allows you to back it into camp spots off narrow entry roads. I've found this beneficial when roughnecking and camping at the Great Sand Dunes in CO where the entry road is narrow/oneway and the camp spots are often perpendicular to the road. Not sure if that makes sense or not. I've towed my father-in-laws double axle Hi-Lo and while it's easier to back up because it doesn't turn as sharply from steering input, it's more difficult to back up in the scenarios I described above.