Wilbur, do I understand your question is this: You had two hot wires from motor home connected to the two circuit breakers on 5000 generator, now can both these wires be connected to the single output on the 2.8 gen?
Is the old gen an Onan 5BGE or BGM, and are you looking at using an Onan 2800 as replacement? Also is MoHo a Winnebago? You should be able to do do that, but the 2.8 gen is only going to have a 25Amp c/b, so you are limited to that amount. You are going to have to do some serious power management with the 2.8. Remenber that the power converter is an often forgotten load that you have to factor in when doing load calculations. Do you not have room for the 4000 Onan Microquiet unit which is only slightly larger than the 2.8?