If you find 500 watts particularly useful, you could probably last a weekend on a couple of batteries. Even for recharging a typical auto battery that's not much more than enough to run a couple of good chargers. If you limit charging to the 8A (typical) rates recommended for discharged batteries to avoid the risks of blowing up the battery you're unlikely to get much useful battery re-charging out of your generator in a weekend.
Higher charging currents can be used for "peaking" on a fully charged battery, or for an initial "quick jolt" on a moderately discharged one; but if the battery is "down" it takes a long time to recharge, and even 8A at start can cause significant heating. Charging "hot" will shorten ultimate battery life. The charger may be "rated" at 8A or 10A but the quick rise in cell voltage during initial charging drops the effective current to half that for most of a safe charging cycle. My current "backup" is a 675 AH Marine/RV battery, that requires, with normal tapering, about 3 days (at 8 amps peak) to come up from where it's close to not being useful to where it produces "nominal voltage" after being off the charger long enough for the acids to mix it up. You can take the watt-hours out of a storage battery a lot faster than you can safely put them back in.
If you have an intermittent necessary use for power at that level, your gen might be helpful to prevent running down your basic batteries, but I've seen few people enamored of such teeny ones who couldn't get along without theirs. A bunch in a neighboring campground did use one at WVA a few years back for their "daquiri blender" but they got drunk pretty quickly and we were able to shut it down after they all passed out - or wandered off. I don't know if the "got the message" or were just too hungover to want another blended drink for the rest of the session.
Minimum useful size for a lightly equipped RV, to operate anything that can't run off batteries for the weekend, is generally at least 3 KW, although some people claim some "benefit" from a 1.5 KW unit.
Virtually no RV air conditioners, or microwaves, - the only common things that would need a generator - will run on less than about 1.5 KW, and few RVs are rated for installation of less than a 2.5 KW. The RV warranty, and certainly the installed appliances warranties, will likely be voided if you try to install a built-in generator less than 2.5 KW.
Something around 3.5 KW to perhaps 8 KW are about as large as I've heard of installed in RVs, mainly because larger ones are too heavy to be mounted without exceeding "point-load" limits. You could do a "DIY" installation, but a reputable dealer/service agency should - in most cases - refuse to install an excessively large one for you.
With the exception of the A/C and microwave, everything electric in most RVs is 12V DC here. The everything electric includes lights, furnace fans, water pumps, etc. Everything else usually runs off propane.
And a friend has, and has used in camp, a Yamaha that I think he said was rated at 700 W(?), and it was deemed excessively noisy by his campmates - so he(they?) shut it off, and he was asked (mostly politely, for a bunch of musicians) not to bring it to the next outing. An equally proud "Coleman solid state" - claimed to be the "quietest thing you can get" - gen owner got the same request from the same crew the previous year. Neither generator was extremely loud, but either was a "foreign noise" that other camp occupants did not accept willingly (with due consideration to the excuses for "needing" them).