Some RVs have a drain valve on their water tanks besides the outlet hose to the RV water pipes to the sinks etc. But some don't. They just have the outlet pipe near the bottom of the tank to use as a drain.
The ones I have seen with a drain have an extra hose running out near the bottom of the tank, with a turn knob that goes along or across that drain hose. You can get at that from above lifting the dinette seat or whatever you have to do.
To drain the kind with only the outlet pipe which is not quite to the bottom, just do the best you can using the "low point drains" and then put your pink stuff in.
Actually, I don't use any pink stuff in our climate, since the worst that can happen is the little water left in the tank will only expand upwards in the mostly empty tank where there is plenty of room when it freezes.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.