Yes, you could just connect the second battery to the alternator but what you'd be doing is essentially creating one big battery. They would charge simultaneously but also discharge simultaneously so when the second battery was flat, so would be your main/starting battery. More amp/hours, sure, but one which would, one day, leave you stranded at your camping spot without power to start the engine the next morning.
The alternator output is regulated so not really an issue and having a 'smart' alternator or not makes no real difference to how multiple battery systems should be set up. The most important thing is to protect your starting battery and while you can buy and install expensive DC-DC systems they really don't do the job any better than a simple VSR which cuts the connection when the starting battery falls to a preset level, leaving enough charge to get going again, regardless of how flat the second battery is.
Like I indicated above, setting up a second battery doesn't have to be expensive. I have a 120a/h AGM battery and charging system that cost just over $300 in total and works perfectly. I use a Matson MA98504 which is currently available for about $110 for the whole kit, including wiring. It's essentially a VSR but a bit smarter and has an override so I could start from the second battery in the event that the main battery failed. With adjustable minimum and maximum voltages and a flashy LED display it's fully automatic and sits neat and unobtrusive, except for the colour!