Kris, If you're primarily working commercial, The 18' isn't going to be too large. I have a 6x 12 single axle Carmate V Nose which is approx. 15-1/2' long from back doors to front of V. So, My V front(not counting the tongue) is almost the same length as an 18' flat front trailer. My work consist of approx. 99.9% residential and with exception of a few driveways which are to tough for any trailer, It isn't too long.
As everyone else is stating, Go with the heaviest axles you can! If you wish to retain the 300gal. tank along with the dual skids, A twin 3500lb axle trailer is going to be pushing the upper limits on your weight capacity. Pretty much the lightest liquid you would have in the tank is water! Detergent and SH as we all know, weigh more than water. The tank alone would max out one axle...
Make sure you do a careful weight estimation of all your equipment and go a little heavy on the number. Keep in mind, Not only do the axles have to support all the weight of your equipment, They also have to support the weight of the trailer itself.
On a single axle trailer, You should have a 300-400lb. reserve on the axle so tandems should have somewhere between 650-1000 pound reserve. A little thing called "Road Shock".........
Mike