Thank you very much for your kind words of encouragement.
A concesssion vehicle such as you are building would fall into the same general category as a manufactured home, motor home, camper or boat. As such they do not fall under the normal commercial or residential codes but rather they are a code category unto themselves.
If you ever get a chance to examine a manufactured home, motor home or camper up close you should find a small plastic shield looking medallion about 3" high either near the main entrance door or near the front of the vehicle. The medallion says "This vehicle conforms to all applicable codes at the point of manufacture"
In turn, given that this is a non-permanent structure most local jurisdictions have regulations that state they will accept the vehicle if it conforms to the codes at the point of manufacture.
The catch 22 here is that the manufacturers tend to locate their production facilities in areas where there are no codes, therefore they can basically do anything they care to do. In fact, there are a couple states that refuse to enact any codes because they feel it would hinder a vital source of job revenue within thier state.
The problem you may run into as a DIY project is that you cannot get the manufactured home compliance medallion, and many states require all food service equipment to be inspected so it is anybodies guess what they may be looking for. With that thought in mind I think you are on the right approach by trying to comply with the standard plumbing codes as much as possible, especially when you consider the standard plumbing codes are in all cases much stricter than the manufactured home codes. Now let us examine some of the basic code requirements that would pertain to your project.
NOTE: all specifications taken from the International Residential Code.
Kitchen sinks are rated at 3 DFU's (Drainage fixture units) per sink trap. (IRC 3004.1)
One CENTER trap may serve up to three sinks, laundry tubs or lavs of the same type in the same room providing the maximum spacing between the drains does not exceed 30". (IRC 3201.6)
The lavatory bowl is required to have an 1-1/4 drain & tailpiece and is rated at 1 DFU. (IRC 3004.1)
Trap arms less than 3" in diameter must be run with a 1/4" per ft. pitch. IR3002.3.1)
Maximum trap arm lenght from trap weir to vent opening:
1-1/2 trap arm 6'
2" trap arm 8'
Branch sizing:
1-1/2 pipe max DFU's: IRC 3 horizontal 4 vertical
2" 6 hor. 10 vert.
Main Vent MUST TERMINATE above the roof (IRC 3102.1)
All fixture traps require venting (IRC 3101.2.1 & UPC 901.0)
From these facts we can determine that your total combined fixture load is 3 DFU for the kitchen sink trap and 1 DFU for the Lav = for a total of 4 DFU's.
The code will permit an 1-1/2 line to carry 4 DFU vertical but only 3 horizontal, so the solution is to install the riser between the two sinks. The three kitchen sinks connected by means of an 1-1/2 triple center waste kit and an 1-1/2 P-trap. An 1-1/2 horizontal waste arm to the riser. The Lav is then connect by an 1-1/2 waste arm with an 1-1/2 P-rap fitted with a 1-1/2 by 1-1/4 reducer in the slip joint to connect to the 1-1/4 tailpiece. (an 1-1/2/1-1/4 trap is a common off the shelf item at any hardware.)
The horizontal lines are connected to the vertical riser by means of a Back to back sanitary Tee. You could then either extend the center of the back to back vertical up to the roof to form the main vent, or you could install a 4" vertical riser with a threaded cleanout fitting on top and screw in an Air Inlet Valve. The air inlet valve is screwed in so it would also serve as the required cleanout for the vertical line.
The 1-1/2 vertical line would then drop down to the holding tank.
If you install the air inlet valve, which may be desirable to prevent having the vent line run up through the countertop, then you would need to install an 1-1/2 vent line to to the roof from the second tap on the holding tank. Rather than go straight through the roof which could cause a roof leak problem you could run it up close to the roof then offset it out through the side wall of the vehicle and extend it above the roof line. On motor homes they commonly install the vent opening right near the roof grab handle rails which then protects the vent from being damaged by the occassion tree limb when manuevering in tight spots in the RV parks.
The codes prohibit gluing dissimilar plastics together and your holding tank already has an ABS fitting so the obvious solution would be to use ABS pipe and fittings to make the drain system. The alternative would be to use PVC for your DWV system then connect the PVC to the ABS by means of a rubber FERNCO coupling.
The discharge line from the holding tank should be a 3" line equiped with a gate valve and a removable cap on the end of the line. You can get the gate valve and an end fitting with cap that is set up to connect the flexible drain line from any motorhome/camper supply house. They will also have a twenty foot collapsable flex line that is used to drain the holding tank. Motor homes typically have a square hollow rear bumper where the discharge line is stored when not in use. You could do the same thing by hanging an 8ft section of 4" PVC crosswise under the bumper with a threaded cleanout cap on each end. You would then unscrew a cap and pull the line out when you need it.
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