Not sure I would call it a mistake as they can be readily installed by a reputable RV dealer. That is what my nextdoor neighbor did when he found out our rig had an onboard genny. He had our local Camping World install a propane Onan genny in his front bay area.
After one of his long trips I asked how he liked the genny and if he used it a lot. He stated he did utilize it but went through lots of propane which he found a hassle and expensive having to refill more often.
He asked how I dealt with the issue, I told him our rig is a toyhauler and that gas onboard generators are commonly installed at the factory. He wasn't aware that we have two fuel tanks installed on our RV, one dedicated for the genny.
In fact we didn't realize that about toyhaulers when we first started shopping them and initially didn't think the added cost of the onboard generator was worth it. We made a great deal on a toyhauler on the dealer's lot that had the genny factory installed, afterwards we are glad we did get this option as we prefer boon-docking and dry-camping.
If we get a rig without an onboard genny (doubt my DW will allow that to happen) I wouldn't have a problem going to parallel units from Honda, Yamaha, or Champion. I like the idea of only bringing a single unit when generator demands are expected to be low.