Terry,
These are all doable things, I suspect you would need to go to a shop. A isolator or separator install is pretty much the same on a SMB as other class-B's. The biggest problem with SMB is everything is pretty tight as far as install space. The easiest thing to do would be to replace the surepower isolator with another. The voltage drop issue of the diodes depends on if feedback for alternator is before or after the isolator. I am not sure about a 1998, myself but I am sure others on the forum may know.
The next option would be an ACR or Automatic Charging Relay. There are a couple that I would recommend that are like the Surepower 1315-200 referenced in this thread, but quite frankly just better. The choice here depends on a couple of things Alternator Size, and if you want't to be able to emergency jump start from house batteries.
The
Blue Sea 7610 SI-ACR Automatic Charging Relay
The 7610 will handle up to 120 amp alternators, so if your alternator is larger than that I can't recommend it. You can wire in start isolation and a indicator LED, but this unit will not provide emergency jump start. Also if choosing this unit take care in choosing the wire size, they generally recommend larger capacity wire for the actual current . You definitely need to replace the existing wires.
Blue Sea 7622 ML-ACR Automatic Charging Relay with Manual Control
The 7620 & 7622 pretty much perform the same, the difference is the 7622 will let you manually open and close the relay, it doesn't cost much more.
The 7622 has more than enough capability for are setups, it also comes with a remote switch, that will allow you to operate in there modes: Disconnected, Auto, and Connected. The most common mode is auto, the switch will also provide LED indication to the state of the ACR and some diagnostic information.
By switching the remote switch connected you can use it for a start assist mode, or emergency jump start.
You can than hook solar solution of your choice to your house battery or if you are interested in combined solution you may want to look at a combined solution. If have installed a couple of these units and to date customers are happy.
KISAE DMT-1250, The 1250 is the 50 amp version, they also make a 30 amp version. The DMT-1250 is a DC to DC charger, it will become a 3 stage charger for your house battery that will run off of your starting battery and alternator. You would not need an isolator, separator or ACR for this solution. While it will provide the isolation and charging functions of a ACR it does not have emergency jump start capability. It does have built in solar, so you could use it as your solar controller. Note: this means you would not need a portable panel with an integrated controller, but one with out.
Hope this helps, try looking into these and feel free to ask more questions. I am hooking one of the DMT-1250's into my own rig soon, and will provide a right up. Feel free to PM also.
-greg