You are talking about a converter, not an inverter... very different products.
How many batteries do you have? If you have just one battery, you'll most likely be fine with a 45 amp converter. If you have 2, a 60 amp converter will be better for charging them both at the same time.
I'm pretty sure that your 12v
fuse block is separate from the Univolt, but it's worth checking before you go off to get what you need for the upgrade and find you need more parts in the middle of the process.
I bought mine from bestconverters.com, but while staying at a state park it may not be convenient to take deliveries. You'll pay more, but there are RV sales and service companies in and around Panama City that will probably stock a decent converter. Progressive Dynamics and Iota seem to satisfy buyers pretty consistently.