There should be a timing tab on the front of the engine
Here is a car version of a front timing cover with the timing tab at upper right in the picture (drivers side on engine). This variation is for the #1 or #6 cylinder for timing.
Some motor home 454 engines had the timing tab at the #4/#7 position and others, like whem2fish indicated, had the timing tab at the #5/#8 position. When the engine is running and you are setting the timing to it's final value, it does not matter which of the 2 cylinders you use as long as it's the proper pair for the location of the timing tab.
Note: If you left the timing cover on the Suburban engine, then you have to use the harmonic balancer from the Suburban engine so that marks will line up.
The mark on the harmonic balancer (macheined line that cuts across balancer edge) should line up to the "0" degree mark (TDC) on the timing tab. Trick is in knowing which one of the associated pair is supossed to be at TDC. If the pair combination was 1/6 (top right timing tab location), then, for simplicity, you want the #1 cylinder at TDC on compression stroke. That means both valves on the #1 cylinder will be closed. The #6 cylinder will be at the end of the exhaust stroke straring the downward Intake stroke (valves partialy open). Once that position is set, you can stab the distributor with the rotor alligned to the #1 plug position. Setup of the TDC is the same for the other cylinder pairs however when stabbing the distributor you want to align the rotor position up to the cylinder on compression stroke (1, 5, 4, 2).
The confusion you may have is that where the original 454 in the motorhome had the timing mark at positions 5/8 or 4/7, the Suburban engine may be at position 1/6. Principle is the same at each location just different pair combinations.