If your house bank (“auxilliary battery”) is dead (nothing is working) we have both short-term and long-term problems. Let's start with the short term issues.

In addition to you having no power in the van, sitting at very low states of discharge is damaging to both lead (Pb) and lithium (Li) batteries. It is important to get battery voltage off the floor ASAP.

1. turn off all loads so your battery and any charging attempts aren't trying to run the household
2. disconnect solar panels, if solar is present, for the reasons given here.
3. apply some kind of charging - any charge is better than none in this scenario - see below
4. when any solar charge controller comes back online¹⁾ reconnect solar panels

The fastest form of charging is by alternator; if you have alternator charging start the vehicle and take it for a drive.
Note: if using a plain isolator a completely dead aux battery can pull enough current to cause resets or overheating. Use other methods first then use the alternator when Vbatt comes up a bit.

Solar, if present, will often be a gentler charging method. MPPT will run normally but PWM charge controllers will be especially weak when battery voltage is lowest.

Shore power chargers are often quite weak (like 10A) but if you can stay on the charger for hours or days the battery will recover charge.

If you have or can borrow jumper cables you may be able to “jump” your house battery from the vehicle with the van running. Relatedly, a lithium jump pack meant to jumpstart the vehicle may bring the bank up high enough where you can charge normally.

1. ensure all charging methods are configured to meet your battery manufacturer's charging specs - voltages, durations, and min/max current
2. ensure all connections are good: battery connections, solar panel connections, grounds²⁾, etc
3. get your power production and power use ratio under control. Make sure you are making power whenever you can.

Yes, your battery may be toast but there is no use replacing it until you get the underlying issues fixed. If you replace the battery with the same problems it will be murdered, too.

Now that the ship is righted again, take solace in the fact that many before you have made these same mistakes before. Vanliving is doing, and breaking stuff, and fixing it.