When we bought our rig (new), I spent weeks going through all the manuals, not only for the chassis/engine/transmission, but the fridge, genny, water heaters, air conditioning, jacks, toilet (actually had its own manual), water pump, inverter/charger, awning(s), slideouts, satellite dish, TPO roof, ..... everydamthing.
I take the rig to Freightliner annually for the chassis/engine/tranny stuff (hard to do myself without a maintenance pit), but do most of the rest of the maintenance myself. Built myself a checklist showing all the different parts, and what's needed and when.
I sometimes get lazy, like last year the genny service was due the same time I took it in to Freightliner, so asked them to do its oil/filter change, but otherwise do most of it myself. 5 years fulltiming and nothing's fallen off yet.
FYI: Some stuff is supposed to be done by a "licensed professional", like the annual propane leak check. Do whatever you feel you must to feel comfortable about that. I figure I'm just as smart (and maybe a little bit smarter) than the dude at Camping World who did the leak check on my rig the first year, so......