WVvan, a battery combiner is an excellent alternative to a battery isolator. I use a similar type (Hellroaring) and can vouch for their effectiveness (ran my yellow top down almost flat the day after I set it up). Mine is set-up with the yellow top as the starting battery and a red-top as the auxiliary. House batteries are separate and charged as previously noted. I’d check the manual on the Sure Power as I don’t believe you can charge the starter battery from the house batteries or vise versa without running the engine. Also, I would run the van enough in the winter to ensure the starting battery stays charged: there’s nothing worse for an engine and drive train than letting it sit idle IMO.
I think that when I get another starting battery (not auxiliary) I may get a true deep cycle one as most have the necessary cold cranking amps to start the engine even after running a portion of the “house” accessories for a while. And even if once and a while I ran it too low I could exercise (combine) the auxiliary battery to crank it over. I’d definitely do it if I only had one hose battery.