Replace old Onan 5000 with box store portable

In theory what you are saying could work depending on how your panel is wired. Box store generators produce split phase 240 VAC. If at any point in the bus bar both phases are present then you would destroy your rig's electrical system. Think bussing both out of phases together. Not a good thing. Another issue is the NEC code requiring a single NG bond in a separately derived system. Many box store generators don't have the required NG bond. Depending on your rig the NG bond could be done by the transfer switch. Without the required NG bond you could have a hot skin condition that can kill you. The last issue is duty cycle. Box store generators are rated for intermittent duty. This means no more than xx hours in every 24 hours. And they usually require full service immediately after a short run time. We have an automatic standby generator system and if it is run for 12-24 hours it requires full service, oil change, plugs filter etc. This runs up the cost significantly. Now there are more expensive stand alone generators such as Honda, Yamaha, etc. that can have a longer duty cycle. (note: they don't have the required NG bond) The Onan generators are rated for continuous duty or what is referred to as "prime power" application such as in an RV. You didn't mention where you planned on mounting the box store generator but they are not designed to be mounted in a confined compartment such as where the Onan was. That is a quick way to overheating the generator and worse carbon monoxide poisoning. Rather than attempt to modify your rig's electrical system it may be better to just plug your shore power cord into the generator until you can purchase a replacement RV generator.