You will not be able to use the SuperSprings # SS35ZR with the Firestone # F56ZR on your 2021 Nexus Rebel 30R motorhome as they both mount in the same location on the leaf springs. If you are wanting to use multiple systems you can combine the Firestone # F56ZR with either the Timbren # THSR001 or the SumoSprings # SS52FR. The Firestone attaches on the front section of the leaf spring while the Timbren and SumoSprings attach on the rear portion of the leaf spring. Personally I feel the... view full answer...