I recall seeing (somewhere) a person in a similar situation, used/made elastic bands with clips on the ends (suspender/mitten clips) and a standard size mattress pad. They pulled the excess material under the mattress. They used the elastic bands under the mattress and clipped to the mattress pad to keep the material under the mattress and keep the pad tight.
I kept this solution in mind because I was sure someday I'd face a similar problem.Maybe it will help you.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.