What kind of SAN disk are you talking about? Fiber channel or iSCSI? You can easily present iSCSI LUNs to a VM by using the software initiator inside the guest. That gives you a lot of flexibility in terms of VMware related functionality (VMotion, etc) while still allowing those LUNs to use native SAN software. If you have an iSCSI SAN I would look into that method to see if it meets your needs.

You could also just create a LUN and make a large VMDK file instead. There isn't much (if any) performance difference between an RDM and a VMDK file, so unless you need an RDM for specific reasons like clustering or SAN software then a VMDK would probably suffice.

What are you trying to accomplish with attaching the SAN LUN to the VM?

When I've seen iSCSI LUNs disappearing in Windows (physical or virtual), the cause is usually either misconfiguration of the MS iSCSI Initiator or some kind of networking problem. Don't judge iSCSI volumes inside the guest by the performance of what you're seeing. I have configured numerous VMs with iSCSI LUNs attached inside the guests for all sorts of applications - Exchange, file servers, SQL, etc. It sounds like the problem isn't really with the strategy but rather with the configuration.

If there is some kind of problem with the configuration of either the SAN, networking, or otherwise, then using an RDM at the ESX level will likely experience the same kind of outages that you're seeing now. I'd try to narrow down that issue before changing the configuration.

Using a VMDK file can also be fine provided you back that up so that if you lose the LUN you don't lose your data. There are a number of tools out there like Veeam Backup or Vizioncore vRanger that are affordable and can backup entire VMDK files pretty easily.