Answering as an emergency vehicle technician I can tell you that the number of wheels on the rear axle depend on how much weight the axle will carry. More weight equals more tires and higher pressure. If the vehicle carries water and a pump it should have four tires on the rear axle. From my viewpoint it is always safer to have more tires on the ground.
Traction, stability, rocks.....are you guy's guessing. The only advantage to duels is if one blows you got another after that a single is far superior. Now just for the record I'm comparing to "super wides" which are a direct replacement to duels, bigger foot print means better traction better stability. Stability is only as good as the width and has nothing to do with number of tires. This technology is relatively new and you all know what that means in the fire service but I am seeing it on some applications. On our new type III, well OK Mike, type IV(2 man cab) I could not get Navistar to recommend but I believe it is available now, the problem was RPM(revolution per mile)and they need to match front and back on 4wd. OTR truckers have found super wides, cheaper, less heat build up and about a 4% fuel savings. And for anybody that has ever been in the mud or sand a bigger footprint is better, now I will say there are exceptions to this and it depends on type of material where 1 skinny tire works better but some playing around with tire pressure will achieve the same results. The one inherent flaw with duels when the space between them fill with mud, your screwed.
We took a new 2009 Dodge dually 1 ton four door cab w/no bed on it. Took the duals off and put super single Mickey Thompson mudders on the rear and Mickey Thompson mudders on the front. "Engineered" a bed on the back w/bins for equipment, 250 gallon water tank, hoses etc., and skid mounted pump. Works pretty good in mud and off road. Old grass rig was a 1975 chevy truck with super swampers all around and we never got stuck in fact we were the ones that pulled all the grass rigs out the just had "regular" tires on. On a regular truck used as a grass rig would never go with duals in the rear, more surface of just one big tire on the ground works good in mud. We don't get alot of wood fires around here just field and grass fires.
We run two Ford 450 with 8.25X20 single tires on custom wheels manufactured to carry the weight (400 gallon of water). This set up allows you to cut thru the mud and keep on moving. Duals in mud just never work as the fill up with mud and debris between the tires.
Our brush units started with the depts first pumper in the 50s. It was a 1930 front mount pumper and has new pumpers were bought it was moved down to brush unit. Later the body from this pumper was moved onto a 1950 Ford Army surplus chassis. It was around for a while before the dept got some money and bought a brand new American LaFrance Little Mo which was a small airport or toll road fire unit. This was on a 4x4 Dodge 3/4 ton which had dual rear wheels. The stories about this unit was something. The old timers told how they would be responding and next thing see a wheel passing the unit. It would break the lugs holding the duals. Well they sold the unit off to a local citzen around the mid 60s.
Then bought a open cab Ford F600 4x4 combo brush and rescue unit. There were bench seats where a hose bed would be. The unit had its draw backs with being underpowered and some said it should had more GMP pump.
After that unit was sold the dept started using pickups with skid mount tank and pumps.