My bus is 50 amp, but I rewired my 8KW generator for 120 volt only. ... so a standard 50 amp transfer switch can still be used. A jumper does need to be added inside the generator between the 35 amp breakers so the breakers don't trip once total load exceeds 35 amps.
If you re-strapped your 8k genny for 120-only, you can be supplying up to 65 amps of current to your mains. I hope you ran appropriately sized wire for that much current; in particular, even if you are splitting the output between two separate hot leads, you will still carry the full total current on the neutral. A "standard" 50-amp transfer switch, for instance, can't be used in this configuration, unless you limited the neutral current (by using, say, a three-pole breaker) to 50 amps, which would thus limit you to drawing only 6,000 watts from the genny.
This is the key reason why RV generators over 6kW are almost universally wired for 120/240; it allows for use of smaller wire and breakers while at the same time remaining consistent and compatible with 50-amp shore service, which is universally 240 by law. (And yes, I know some campgrounds break this law routinely, before anyone jumps on me for saying that.)
As to the OP question, I don't have a clue why anyone would downgrade a 50-amp system to a 30-amp one. You can always use a 50-amp coach on 30-amp power, but you can't squeeze any more power into a coach that is only wired for 30-amps no matter what you do. Besides which, most of us are required by code to have a 50-amp system (any more than 5 circuits, or two thermostatically controlled appliances requires 50 amps).
We get around the biggest nuisance of the 50-amp system, the "elephant trunk" power cord, which is heavy, hard to fake, and stiff as a rod in cold weather, by carrying a completely separate 10-gauge cold-weather-rated cord for cases where only 30 amps are available. We'll use the 10-gauge unless we need to run two air conditioners; at that load, the voltage drop on the 10-gauge (which happens to be 50' long) becomes a problem, and we switch to the 6-gauge four-wire with a "dogbone" on the end.