The yellow residue is a sure sign the unit has a leak and shouldn't be used. A strong ammonia smell is another tip off that the coil unit is bad.
Had a bad one years ago and helped a RV tech replace it. Not all that complicated and I've since watched videos and read postings that detail the work and think I could do one if I had too. Since you're handy, should be a breeze for you.
I haven't used either of those outfits but you can search the various RV'ing forums for the names to see what comes up. I always check an unknown supplier by going to google and typing, "complaints about companies name" and though that doesn't always show something, very often it leads to some reviews.