You bought used, correct? Pull the 13.5 apart from the roof and clean it. Chris Bryant has an article
HERE. It's incomplete, broken links to the pictures, but it tells you what to do.
I'll venture that your 13500 IS adequate for your size coach and will keep you cool if you keep it clean.
The intake filters too, of course. I salvaged a 20 year old Coleman Mach 13500 that I thought I was going to have to replace, following Chris' article.
While the shroud (the pretty finish one) is off, make sure that (if it's Coleman) that there's a foam baffle running vertically along both ends of the Condenser (rear Coil). If missing you can make them out of foam pipe insulation or a swim noodle. Again, this thanks to Chris Bryant.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB