Hello and welcome to the group. I believe I am only 1 out of 2 members of this group that has this factory option. Mine is propane I would be more than willing to take photos of it for you.
The major thing that Sunline did while building my unit was installing the interior hour meter with on/off switch, the wiring/plug for connecting the unit to gen power VS shore power and building the "compartment"
When I was ready to spend the money, I had a local dealer install the Onan 3600LP unit and complete the install. They did the propane and electrical connections when they installed it.
Being single, I went with an installed unit, to eliminate one less thing to be loaded/unloaded. But, now, I believe IF I had a chance to "do over", I would opt for a stand alone gasoline unit. Cheaper $$$ all they way around, and can be used without the trailer.
Sunny #18 2003 F-344SR #8157
Sunny #19 1997 T-2653 #5485
1999 Ford F350XLT DRW Crew Cab, Long bed 7.3 PSD
2002 VW Jetta TDI.. AKA: Kitty's Kruiser