If you over insure your coach you will automatically put yourself at odds with the insurance company if you have a loss. If the price you wish to insure it for is above the current market value for your coach you must document and prove why that is the case. It is coincidental but there are "twins" to our vintage coaches in the market and their selling prices are going to be the value of our coaches.
Your premium is based on the insured value to an extent.
If you under insure then you are potentially at risk to lose your coach. A minor accident may allow an insurance company the opportunity to write you a check for the whole coach, take it and flip it to someone who will buy it as is for the insured value or maybe a little less but not for a difference that is more than the cost of repairs. You get the insured value, but it is not enough to replace the coach.
I suspect the financial turmoil now gets everybody edgy and insurance companies probably do not want someone over insuring a coach and then driving it off a cliff so they can make a few bucks.