Sunnysideup solar for inverter, solar and lithium installations, Stu did a great job for me and reasonably priced. They will also come to your place to do the install.
Not van specific, but maybe Cotswold Outdoors. They've got a good range of outdoorsy stuff, and they are used to linking up with organisations for discounts, as I've had them previously via YHA, Scouts, Archery GB, National Trust, CAMC I think, etc.
There was a discount with Cotswold outdoors. It stopped around 2-3 years ago., I believe the supplier pulled the deal. As you say, they’ll accept CAMC, CCC - and blue light. Each of these memberships can be verified via an annual card, unlike the Cali Club. I suspect that this could be the issue.