A couple thoughts.
I started with some of the aesthetically pleasing air filter units that had good wife approval factor. When it was time to replace the filter elements in the fancy units the replacement filter element price had very low wallet approval factor.
HEPA is for sick people. On the MERV scale HEPA level filtration starts at MERV 14. The MERV scale was developed by ASHRAE, an HVAC industry group. If you have good MERV 13 filtration in your home and are still having issues, then look at HEPA. If you are an organ transplant recipient, sure, maybe start with HEPA level whole house filtration.
As above, the various symptoms associated with radon so far in this thread are concerning for lung cancer already having developed in the victim. However, there is another possible explanation. The RV had a ceiling vent fan, so good air turnovers. The house has no HVAC system (like mine), so relatively poor air turnovers. Besides the animals, how much carpeting do you have?
If you go reading up on carpeting, you will likely find they give off nasty chemicals for a year or two, and by then they are saturated with dust and pollen. If you want a dirt floor, put in wall to wall carpet. If you want clean air in your home, take out all the wall to wall, and buy the display version of various throw rugs that has been offgassing into the flooring showroom on a hanger system, but not picking up dirt. Wash the throw rugs on the driveway every spring with soap and a garden hose...
With all those animals - and probably some carpeting - the sustainable way to git 'er done is (as previously mentioned) is to duct tape a 20x20 furnace filter on the intake side of a 20 inch box fan and let it rip. I am also active on a woodworking forum, this is a probably safe step for most home owners in most situations. There are lots and lots of woodworkers who have been doing this in the garage to keep dust down in the house for decades and decades. You may purchase an infrared thermometer if you like to keep track of your fan motor's temperature while it is sucking through a filter. If you find more than 1 degree Fahrenheit temperature change between ambient air and motor temperature in this scenario I would very much like to hear about it.
I find the Lasko fan (home store team blue) is noticeably quieter than the black sexy similar item at home store team orange. For MERV 13 filtration you are looking for team orange "level ten" filters. On the 3M filtration scale, both the 1900 and 2200 Filtrete fall inside MERV 13 parameters. I buy the 3M 2200s by the dozen from that big online place, size 20x20x1.
With a geriatric long haired cat and a hard working woodstove I go through about 3 filters per year on one 20" Lasko.
My existing thread about indoor air quality is here: https://www.hearth.com/talk/threads/homemade-air-filters-good-indoor-air-quality-cheap.188986/