Gen sets can have electric pumps as they were introduced for enclosed generators, due to ozone causing deterioation to the rubber diaphrams of mechanical liftpumps. I was told this by a dutch vetus main dealer. I concur with your view about onan gensets, a very quiet piece of kit, particularly when fitted with halyard exhaust systems.
I dont know for thornycroft, but the usual time is every 200- 250 hours for many manufactures of marinised engines. when we did a lot of inland french canals etc & the boat was underway for 3 months, I use to do an oil service at 6 weeks. engines were 2x mercedes benz om314. (a direct injecion 4 cyl diesel of 70s build) oils today are of a higher standard hope that helps