Other than the new ones having a warranty (for what that is worth) the costs and parts prices are similar. Current shop rates are $185 and up.
I have found a mobile mechanic to usually be cheaper and more effective. As far as reliability on age, I think the breakpoint is around 10 years, after that things start to wear our as they age.
Other things like tires and batteries are every 3-5 or so. We recently purchased a 2014 Itasca Meridian knowing there will be a host of small things to be dealt with. Our other RV is a 2016, we replaced batteries an tires about 18 months ago, other than regular maintenance it remains trouble-free.
Aaron