You don't literally need a negative buss bar. The issue often debated is whether to rely on the van chassis as your negative return to the battery from your various 'loads' or to have a separate ground wire return for each load (not providing a ground wire return is a common technique employed by auto manufacturers as it allows them to save by not putting in additional wiring for the negative lead/return). Our choice, and Gary's recommendation we believe, is NOT to rely on the chassis for ground current return, rather, to close the circuit 'back to the battery' with equally large negative conductors from your various loads . . . equal to the positive wires going to those loads. This does not mean you shouldn't ground the negative to the chassis at some point. But for technical reasons (often referred to as "ground loops"), it is not a good idea to ground to the chassis at more than one location. Therefore, we'd recommend you select your house battery mounting location as the point, and only point. to ground your negative to the chassis. (An alternative would be to ground where your vehicle battery is grounded, particularly if you're going to tie your vehicle battery/alternator into your house battery system for charging purposes. In this arrangement you'll maintain a single vehicle ground but, instead, it will be at the location of the vehicle battery.)
Looking at this from another perspective, pretend you don't have a Promaster, but are wiring everything out in your driveway. You'll need both positive and negative wires from your battery to your 'loads'. Again, make them all the same gauge. Now, pretend, further, that you're going to take all this wiring and place it in your van. It would operate correctly as re-positioned in the van without any further connection to the van chassis. But it is strongly recommended that it be connected to the van body, again, at one location.
As to a 'formal' negative buss . . . remember, you don't need any fuses in the negative side . . . so the 'so-called' negative buss is nothing more than properly 'connecting' all the negative wires to the battery - - you can do that anyway you want, you don't need a special 'buss bar'.