Wolfe10 and Jiffyjet2 are on target with their replies. I weigh and analyze all types of RVs , so I have dealt with many concerned drivers about handling issues. One of the components of our analysis program is the wheelbase divided by the total length of the coach. The resulting number should be .54 or above to eliminate design issues that can cause "porpoising". Now that doesn't mean a .54 and above will never experience this problem, if the coach is not loaded properly, or suspension problems may exist causing handling issues to be present regardless of the WB/Length ratio. It also doesn't mean a coach with a .53 or lower ratio will always have issues, it just means there is a possibility that design may contribute to the problem.
This may be valued information, but that wasn't exactly your question. Your concern sounded like it dealt more with the weight of the rear engine diesel on a small wheel base. That would fall under another element of the analysis that deals with the % weight on the front and rear axles. Typically, the front axle should carry between 22%-38%, leaving the remaining 62%-78% on the rear axle. The rear axle will have a rating considerably higher that the front, which should allow the coach to achieve the close percent capacity result Wolfe10 mentioned. As far as I know there is no industry standard on how close the percent capacity should be, but I the closer the better would be good advice.
Beaver and Country Coach both made some outstanding coaches, but I have heard your very complaint from owners of the shorter models.
If ride and safety are major concerns, which they should be, BY ALL MEANS, have the coach weighed and analyzed before you buy it. "Weigh before you pay" is good advice anytime you are considering an RV purchase.