Protecting inheritance: What happens when MIL's spouce passes away?

“Her name was on the title to the house” normally means that the house was in both names as a joint tenancy, and the survivor automatically owns the whole. That applies to married couples in almost all cases. You just produce the death certificate, either immediately or the next time there is a dealing with the land. It is possible (but very unusual outside of business dealings) for the title to be as ‘tenants in common’, in which case M’s share goes according to her will or by local intestacy rules.

Get a copy of the title and take it to a lawyer to explain – or even just to find a copy of the title, if this is all just ‘memory’.

It is time to see an elder law or trust and estate or probate attorney. A death has consequences and things need to be handled. The husband was, I assume, still married to the wife. Her estate, without a will, will be divided according to the rules of the estate.
Take all deeds and ownership documents and wills if any with you and see a probate attorney. Someone in family (husband if he is competent and capable) will be appointed as "administrator" of wife's estate, will take care of everything with the help of the probate attorney.

In matters like this state laws matter. As does the fine points of deeds and titles. This isn't something for a Forum of strangers to guess about. This needs to be legally handled.

Best wishes.

Did she not have a Will? Does the RV have insurance on it where it would be paid off if something happened to her? The company should be made aware she has passed. They will repossess it if no one can take on payments. It will then be sold to recoup any money they are out. If no will, someone needs to go to probate and become her Administrator. This will give the person the ability to buy and sell. No Will means the State steps in. Dad will probably get any of her assets as the spouse. Her children maybe getting a % and that may include the house. At that point, ur FIL can then have the deed put in his name and the childrens if they are entitled to a % of her half.

If there was a Will, it may have said "what mine is yours" which means FIL inherits. He then can change the deed to his name only. He may need to get a short certificate from probate to handle MILs assets not in his name. Just check with Probate how this is all handled.

Did she have a will?

Why not sell the RV and avoid such a mess?