Hey there @Seras and welcome to Outdoorsy!
So I did a quick search based on Colorado and found that Class B's are getting the most requests/bookings per listing and Class C's are a close second. Campervans are also doing pretty well but would be a 3rd place to the other two. Hope this helps!
This is where it's all up to you. I'm sure you and the renters might feel a bit more peace of mind if you were able to find something under 100k miles on it. There will be an initial verification/checklist and then a cyclical 90-day inspection after that as well to make sure everything is in good working order.
I recommend looking at teardrop trailers like ours: www.droplet-trailer.com
This type of towable has the best return on investment. DROPLET -> 16k purchase for $100/night. CAMPERVAN 100k purchase for $250/night.
Also DROPLET promotes your rental directly on the website. www.droplet-trailer.com/booking