sorry, there is a lot going on in this thread and i am by no means an engineer, just a service technician. i can diagnose and fix a generator and an ATS but most things beyond that is up to the electricians or project managers.
diesels pretty much need a load on them, meaning that when you do run it to exercise it you'll want it to do an actual "pull-the-plug" test, as in have the ATS transfer from normal (utility) to emergency (generator). modern diesels in generators want between 50-80% of engine load capacity on them to avoid wetstacking exhaust, and this is especially true in the smaller diesels that kind of "avoid" tier-4 emissions standards, as in no DEF system or needing regen cycles. because of this their wetstacking or at least, well, stinky exhaust is more noticeable.
but for fuel issues it is pretty difficult to avoid. things just tend to wear, i guess. all i can say is for any of the rubber o-rings on fuel lines i use 24c grease to lube them (cheap and its on hand on my service truck) and have never had an issue with the same engine again. it will take years for the o-rings to begin to fail, i think it's more of a time issue than an hour-meter one. rather than fuel leaking out anywhere i typically find the unit has just lost its prime and will crank but not run, and can take multiple crank cycles to actually reprime itself to run, trying to compensate for pulling in air through wherever the bad seal is. sometimes it'll just kill the battery and other times it'll go down on an overcrank or locked rotor fault and i'll get a call. when customers do that i think that definitely makes a negative impact, you don't want these pumps running dry and pulling a whole head of fuel, you would want to reprime the system yourself manually. sometimes it'll crank and turn but then shut down within 2 seconds as it already lost fuel pressure. typically, it'll display the low oil pressure fault when this happens
otherwise you're going to want to keep some spare fuel filters on hand, and never fill the filter yourself as a shortcut when changing. Put it on dry (except oil the mating seal obviously) and pull fuel through it to prime. these modern diesels do NOT like dirt getting in and it is not worth the risk of introducing any particulate. after priming to the fuel filter and sealing either the bleed screw or the fuel line back in place that you used to prime, there is usually either an electric pump or a little push pump to continue to force fuel up to the injector pump from there, and whatever little air is left isn't enough to hurt the pumps.
beyond that, fuel stabilization for anti-gelling in the cold and general anti-algae treatment and still expect to drain the tank and have it be refilled every couple of years to keep it fresh.
nothing about a fuel system in a diesel (up to the injector pump) is complicated and easy enough to figure out, especially with help from the internet. i'd consider all of that pretty much DIY for anyone interested in saving the money from calling a technician out.
otherwise you encounter the regular issues with any engine - oil pressure sender failure, EGR valve stuck, thermostat stuck, coolant or water pump leak, belts, you know, the normal "consumables". it is rare for a regularly loaded diesel to have any bigger issue until it gets pretty high in the hour meter count.
also, you wouldn't really have to worry about any of this with an LP unit. maybe a stuck regulator diaphragm is the worst issue you'll find, an easy fix.
Cool, lots of great info...thanks