Well I dont know a lot about horizontal tanks- Ive only had 1, on a camper van -hard mounted & it was filled in place. That said looking around Im not sure ALL horizontal tanks are meant to be filled horizontal- but I only looked around briefly. Also not sure on the price recertification justification (though pretty sure thats not the motivation to convert). However my last 2 recerts, a year apart was $10.00? A new replacement horizontal tank considerabley more I would think.
IF you can get a tank thats designed to be refillable horizontally I would think more likey done by techs if the tanks could slide out at least half way. The inside semi enclosed cabinet seems 'might' be issue.
In my eye the mechanical part, assuming stacked on tray will pass door height, the easy part. Modifying the tray to accept bolting bottom tank and accepting /designing saddle for second tank, so both tanks mounted to tray. Verifying affordable tanks can be purchased to hard mount and can be refilled in place take a bit of research.
Im going to town tomorrow. Ill ask at local RV shop if they would refill on board tanks or their thoughts on your desire. They also work on RVs so might have some valid input. If I make to other side of town-theres another shop-thats all they do is refill tanks-simply get their opinion on refilling.
