I buy and use Filtrete sheet-type filter material and make/cut mine to fit the return air grill, works very well. Also you may buy 2"X10" filters that fit under your heat registers, made specifically for this application; an added result is, these filters prevent dirt and stuff from falling into the heat duct-work.
Suburban might not like it, but they worked great in our 5er for 8 years. I now use them in the MH LP furnace registers. My basement air came with a 16"X20" pleated filter installed, which is changed monthly during summer months.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"