I have bent 3 different axles with my trailer and the 325 gallon tank full. It is a 14' long trailer with 2 hot water skids on it.
A lot of times it is the potholes that bend the axles, not really the speed of driving in my case.
The way that the trailer guy explained it to me was that when you hit a pothole, he force is multiplied, it is not just the weight on that side of the trailer.
He also told me to never have the trailer more than 75% full to capacity as you need some room in case you hit potholes.
I was going to go with the 5200 pound axles but at the last minute I decided to stay with the 3500 pound axles as I will have less weight on the new trailer (fleetwashing trailer) and do not plan on hauling water as the current customers have plenty of water. I will be buying a box truck soon, just could not swing it by the end of the year and that truck will have a large water tank so when I get the customers that do not have water, I can haul it no problem.
Rember that the skids will be about 650 to 900 pounds empty, then add fuel to both tanks, water in the coil and then the weight of the trailer and then a partially filled water tank besides hoses and chemicals.
With 2 hot water skids, large tank, chemical tanks or drums, tools, etc..... I would probably go with the 5200 pound axles as the trailer will weigh about 800 pounds heavier with larger beams for the added weight capacity and that is 800 pounds less that you will have so you will only be able to put about 7500 pounds onto the trailer itself more or less and try to leave some weight off the trailer for potholes, especially in parking lots where they can get big and deep.
If you will have chemical drums on the trailer, the brightener can weigh up to 650 pounds and the soap a little bit less, just depends on how much you want to put on the trailer.
Each time I changed out an axle it cost me about $600 by the time you put brakes on the axle (brake hubs, brake assemblies, etc...) (I only get about 9 months to 1 year out of trailer brakes), the axle itself (with hardware), 1 hour labor, and 2 new tires (bent axles will chew the tread off the inside of the tires)
it adds up quick.
Hope this helps.