F550 Off-Road payload capacity

I hope this is in the right forum, I haven't had much luck finding an answer anywhere.

I own an oilfield dust control company. We spray products on the lease roads to help keep the dust on the ground and not in the neighbors homes. Having a larger truck like a single axle tractor makes it almost impossible to do our smaller jobs (arenas, ballparks, etc) so I'd like to upgrade to either a Ram 5500 or an F450/550.

We just finished up a job where we had to do 78 truckloads of product, utilizing our 500gal tank, and really lost our butt because of how long it takes to refill each time. I'd like to have a tank that can carry 1600 gallons. At 11.5 #/gal, that puts the payload at 18,400 lbs not including the boom, tank, pumps, hoses, fittings, etc. I know this is over the capacity of the trucks rating. I know it would be illegal to haul that much on the road, but for offroad low-speed spraying, is this still too much weight for the truck? Its never rough areas, always smooth since its just been graded and compacted. I know a regular watertruck is ideal for these kind of jobs but considering they dont' make up very much of our business, I can't see spending the money to have a piece of equipment that only gets used for a handful of jobs per season. An F450/550 would really be the ideal candidate. Even if I only filled the tank to the legal limit for smaller jobs, having the extra capacity for bigger jobs would really help a lot.

Any advice?

Edit: (To clarify, we wouldn't overload the truck for highway use, only when we are filling the truck from our bulk tanks off-road)

 

We have a trailer now that has a 2500 gal tank on it and it NEVER gets used because its such a pain in the butt to maneuver. The trailer works really well but its limited for what we want to do. If we are covering large areas like long roads, it works great but trying to get up in the corners and around buildings backing a trailer in to spray is a LOT of work.

 

According to the Ford website I can get a "payload upgrade package"

That still doesn't tell me what my payload would be without knowing the weight of the truck, right? To find out what my payload would be I would need to take 19,500 - Truck Weight = Payload Is that correct? That only gives me 12,XXX Payload according to them.

That would give me enough capacity for 1043 gallons of product. So a 1000 Gal tank plus pumps, hoses, the steel flatbed, driver, fuel, spray boom and washdown tank will still only allow me (legally) about 900 gallons of product (10,350lbs).

900 Gal of product would be great for the majority of our jobs. However, when Im offroad doing a long road job I'd like to put about 18,000 lbs of product on the back of the truck. My concern is that I will tear up leaf springs or be dragging truck parts or wearing out bearings. In your opinion, what kind of weight can the truck carry off-road at low speeds of around 7-9 mph without tearing things up?

Reference 2013 F-550 Payload Capacity Chart:
http://www.ford.com/commercial-trucks/chassis-cab/specifications/payload/

But on THIS page it says the 5th Wheel Hitch Weight limit is 24,800
http://www.fordf150.net/specs/05sd_specs.pdf

Im confused.

 

To get into a new truck like a topkick, sport chassis, or single axle truck that has 4x4 is a small fortune... Just out of reach for us.