How much upkeep for portable Tri-Fuel generators
Note: This answer will be more generalized, than specific to generators. Also, I have a small engine repair and service certification from Briggs and Stratton.

All internal combustion engines will need regular service several systems. The most common of which are:


Lubricants
Air Filter
Fuel Delivery
Ignition


These things needing service will have vastly differing service schedules, depending on things like their duty/use types, sophistication of manufacturing, fuel deliver (carburation v. fuel injection (wet) v. direct fuel injection), type of fuel, age of motor, effectiveness of seals and so on.

For instance, it's not uncommon for a modern, fuel injected car motor, to safely go 5000-7000 miles before needing an oil change, and might go nearly 100k miles on single set of spark plugs (my wife's old CRV).

However, for things like generators that get only intermittent use, these problems follow a different schedule, that will be unique to that motor and schedule of use. Only being used perhaps once a year, makes generators particularly difficult to maintain and diagnose problems, since you may only find it when you need to use the generator. Many fixed installation generators remedy this with a weekly test run cycle, and built in diagnostics.

Liquid fuels like kerosene, gasoline, ethanol, and diesel all have different degrees and problems that can plague them, mainly in storage. Water in fuel, and evaporation of fuel is especially a problem for carbureted motors, which are effectively open systems. Thankfully, this is eliminated (mostly) in fuel injection systems, which are not open to the atmosphere, subject to evaporation.

Moving to a gaseous fuel (propane, natural gas, etc) similarly has much less of a problem (if none at all) with this issue with evaporation and deposition of the heavier components. However, liquid fuels usually provide some degree of lubrication to things like valves, and to a less extent the piston in the block. A gaseous fuel has no/negligible lubrication properties.

TL:DR All engines require service. Gas powered require the least maintenance, but can be more expensive initially.

Personal Suggestion: Go to a fixed installation generator on gaseous fuel, automatic cut-over on power failure, hopefully having automatic diagnostics and a regular self test cycle. Then you just change the air filter and oil when it tells you to.