First, Yes you can call your mortgage company anytime to check your mortgage balance and or payoff amount. Be sure to ask for a "payoff" amount if you want to know exactly how much you will owe if the property is sold. A payoff and the mortgage balance will most likely be different amounts.
Disclaimer: I am not an attorney, so don't consider this legal advice. Consider this as merely informational. You would be wise to contact an attorney to resolve an issue with the doublewide not being totally being on your land.
If the doublewide is not setup totally on your land (I'm assumeing that it is not a permanent foundation) that is being sold it will most likely be difficult, if not impossible, for a new buyer to get any kind of financing.
If the purchaser is paying cash and there is no financing involved you certainly won't run into the buyer financing problem.
In Kentucky it is the sellers responsibility to disclose known defects in a property to a prospective purchaser. An encroachment (doublewide partially on someone else's land) like the one you've described would most definitely be labeled a defect.
There are several different remedies to resolving the encroachment problem.
1. The most obvious would be to move the doublewide. This may seem expensive but could ultimately save you money in the long run.
2. Ask the neighbor to sell you some land to cover the area that is encroaching. This would require a survey and possibly approval with your city/county planning commission. It could be costly; Cost of land+cost of survey+planning commission approval+title exam+title insurance+deed prep and transfer=?
3. Ask the neighbor to grant you an easement for the portion of land that has the encroachment. It would still cost a survey+approval+easement recording fees=? but should be less expensive than purchasing the effected land.
However, If you own a doublewide that partially exist on a easement it may be difficult to sell or may not be worth as much to a purchaser than a property that did not have an easement.
Good luck, I certainly hope everything works out.