Only real disadvantage of the diesel is initial purchase price, about $4,000 more than propane.
As far as advantages go, you'll hear some 'I'll never have a propane generator' comments and they may have merit with respect to commercial work or a large Class A RV, but in a Class B or small C... not so much. Commercial work and private use are different applications and have different answers.
In terms of longevity there's not going to be much difference between the Onan 3600LP (propane) and 3200QD (diesel) because they are both air-cooled 3600-rpm engines. The typical water-cooled, 1800-rpm diesel gensets found in Class A RVs will have a longer life, but that's not what we're talking about here, apples and oranges.
Runtimes also won't be all that much different. The diesel generator can only use the top 3/4 of the fuel tank (about 19 gallons in a Sprinter), and even that assumes that you will arrive at your destination with a full tank. Arrive with half a tank and you may only have 7 gallons available. Diesel gensets uses less fuel so that is an advantage, but bottom line depending on how much fuel you have available in your main diesel tank and the size of your propane tank you may or may not have longer runtime with the diesel. Also while runtime can be very important in a large Class A where you might need to run the A/C all day long to keep the coach cool when driving, this is almost never necessary in a Class B/small C as the dash air is usually adequate. Plus, a large Class A might have a 150-300 gallon diesel tank providing a significant fuel capacity advantage over propane, not 26 gallons as with a Sprinter. Again, apples and oranges.
The one factor that remains is that a diesel genset means looking for propane less often (if you use the generator a lot) That's not usually a problem in the US so not really much of an issue, and whether it's worth an extra $4,000 to you for the occasional convenience is a personal decision. IMO you would have to use the generator a lot for it to be worth it, and in a small RV in a non-commercial application you probably won't be unless for some reason you frequently need air conditioning for very extended periods while stopped. Most everything else is best done with an inverter so solar may be a better place to put the money.
So, if you do insurance adjustment work in disaster areas or do pet grooming (you know who you are
then you probably want a diesel. For private use it's much more of a mixed bag due to the high initial cost of the diesel vs. the relatively few, really only single advantage in a small RV.