If you tried to turn a regular (tax deferred) inherited 401K into a Roth? You'd pay how ever much taxes that amount ($90K?) depending on your taxable bracket. If you're already in a high taxable bracket, you'd end up having to declare all that 401k as part of your taxable income for that year, and lose a large chunk of it to taxes - so doesn't sound like a great move if you're trying to keep as much money in your pocket as possible.
There are ways to move smaller chunks over called a Roth ladder from a traditional (rollover), but it's something you'd need to do your homework on to make sure you do it in the most tax efficient manner.
RMDs are not going to be terribly large due to your husband's age and the size of the IRA (likely under $3K per year), so the taxes owed won't be anything as much as converting the whole thing in one year. You should probably look into converting it to a traditional (inherited) IRA, set up the RMDs and then figure out how much you can pull out to do a backdoor Roth conversion each year (if you're eligible - your income or tax laws may have changed to mess with this handy move).
https://www.marketwatch.com/story/should-i-convert-an-inherited-retirement-account-to-a-roth-2017-09-11The only thing you absolutely must jump on is making sure that whatever account you decide to turn it into, it is labeled correctly as an INHERITED account so they don't force you to take all of it out in a 5 year period. The inherited IRA account is a pretty decent account even with the RMD as you have no penalties for taking whatever amounts you'd like - you could take the minimum, or you could take 10, 20 whatever... as long as you pay the taxes owed if it is a traditional IRA.
And finally - wanted to say I'm so sorry for your family's loss.
