There are a couple of good reasons to isolate your house battery from your vehicle, but the one that gets me going is I don't like being stuck.
You can charge batteries in parallel. When you do, the battery charger guys will tell you to use the same batteries of the same age and same size. This is because they share the charge circuit and if they are seriously mismatched you run the risk of damagine the weaker/lesser battery.
On the discharge side of things, when the batteries are connected in parallel they both get discharged. Here's where getting stuck comes into play. Discharge your car battery and the car won't start.
Not good.
There are lots of ways to isolate the house from the car, ranked from easy/cheap to not-so-easy/expensive.
1) Don't connect your house to your car.
2) Unplug the house from the car when you camp.
3) Use a relay to disconnect automatically. Not a starter solenoid; these are not intended to be energized for long periods. Just use a cheap automotive relay. They cost under $5.
4) Use diodes. As mentioned, they exhibit the nasty .7V drop and can impact charging of the house battery, which will limit battery life. These cost ~$30 or build you rown for about $10.
5) Use a FET type isolator. Unlike diodes, no 0.7V drop, but they do have a small series resistance. This is normally not a problem, though it does exhibit a voltage drop under load. These cost $50-100. Or you could build your own for $?
In anything at all, perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away.
-- Antoine de Saint-Exupery