First you’ll want a diode with a lot more maximum current than 30A for a 32A system, I’d typically use a 60A or higher diode. Second you want a reverse voltage a lot higher then the 14 to 15V maximum the circuit can have, I’d pick 30V.
Here’s a search for in stock, 60A and greater diodes, all of them have a 40V or higher maximum reverse voltage rating.
https://www.digikey.com/short/zw3wv2
Do be aware that the diode will drop 0.5 to 1.1V depending on the current. You will also need to handle the heat generated when 30W (30A @ 1V) is dissipated in the diode.
A relay will have a much smaller voltage drop and very little heat generation. Here’s a selection of automotive grade relays that should work.
https://www.digikey.com/short/zw3nqm
This relay in particular should be easy to use since it has a mounting tab for bolting to the RV and has standard spade lug terminals commonly used in automotive connections.
https://www.digikey.com/short/zw3njt