Prepare to (1) be flexible on size- a little oversize just requires cutting the
fiberglass, and (2) redefine affordable- RV
windows are just plain expensive.
Definitely scour RV salvage yards, local and/or online. Shipping is costly, something to consider when shopping online. Check nearby Craigslist.
Worst case, you may have to cough up for new windows or tackle
fiberglass patch work to make the opening smaller.
I’m afraid it’s one of those things people expect to be simple when they take on a project, but it’s not.