Another question CHR$? I noticed you sourced a fin and tube transmission cooler. I've heard that the fin and tube is not recommended anymore for trans cooling because only the fluid on the outer wall gets cooled and the fluid in the middle just rushes by and doesn't get very much cooling. I think the more recent transmission cooler is the stacked plate model which acts more like a automotive radiator. If that is actually true about the fin and tube type it seems that it would not transfer heat to the interior of the RV very well. Maybe you could get more efficient heating from the newer style.
Stacked PlateFin and Tube