I personally don't know of an easy way. I just put the new mattress on, guesstimate how much heavier it is along with gauging the lack of "push" from the existing struts, pull one to get the OE weight then order a set with my new "guesstimation" of how many lbs. I need. I will say that I was wrong this last go round so now have a nice set of new struts that weren't as strong as I wanted under the bed.
I removed the king bed that came in this one and had it cut down to a res queen. I added a heavy res queen mattress . Here is a link to the struts I ordered for it;
https://www.rvupgradestore.com/RV-De...-p/20-1188.htm