RV Consumer's Group thinks that the ratio of wheel base length to overall length is a good indicator. Here's a bit of into I found:
Excellent - 58% and above. Any wheelbase in this category should maintain a straight and steady course under most adverse driving conditions.
Good - 54%-57%.
Fatiguing - 51%-53% is considered fatiguing to drive because it will most probably not maintain a straight and steady course under normal highway driving conditions with side winds or heavy truck traffic.
Dangerous - 50% or below is considered dangerous at any speed on any road.
For the Sunseeker 2650S, the ratio is right at 58%.
For my 24' Winnebago, the ratio is just shy of 54%. I wasn't satisfied with the way it drove so I added upgrades. I'm not saying the upgrades made it drive like an SUV, but it's acceptable.