Just to close the loop on these issues I was having, it seems the shower works pretty well if we start with the hot water valve on all the way, and then just adjust the temp with the cold valve (as someone on this forum suggested)!
And the heater vents looked pretty clean when looking into them with a mirror, so don't think any varmints are in there. And my dog didn't seem to be too concerned with the vents on our second trip, so all is well (except see my inquiry elsewhere about wanting to replace the dash radio on this 2003 Itasca Chevy chassis!). Thanks to all!
