You're looking at 1200l tank on a 40' boat - so range anxiety will not be a thing on the Trent.
The tank on their current boat is >1000L, I'm pretty sure the 40'er is north of 1500L.
However, at 100+L/hour per engine while covering lengths of some of the lakes while on plane, and probably 20L/hour per engine even at low speed cruise, that can go fast. But yeah, it would still do a 3-4 day Bobcaygeon, Fenelon Falls, Kirkfield, home trip without needing fuel.
When in your home harbor, call a fuel guy - mobile filling trucks work small marinas (the ones that server farms), and they are much cheaper than marina gas.
Their current marina has a blanket no fueling on the dock policy, jerry cans or otherwise. Presumably so they can sell their own gas there. May also be insurance related. The advantage of a diesel would be that they couldn't stand on that when there is no other alternatively. Alternately, sail over to the public boat launch and tie up there and have the fuel truck meet there.
They typically fill the boat in the spring at the reserve - the trailer guy charges 1 hour minimum for a trailering job that takes 10 minutes, so they started including a trip to the reserves pumps as part of the spring routine now.