My old man use to have a extra bay in the truck for a second battery for the trailer and would swap it out .as needed. There was also a switch that allowed him to isolate either battery or combine them.
You can buy portable 100w kits or put the pieces together separately for around $200. 200w for between $350 and $400.
LED lights are cheaper than ever and really do use much less power.
When we started boondocking with a single grp 27 we could go 3 days/ 2 nights even with running the furnace at night when it was in the 20s outside. May have gone a third night but we extended it to a week and I didn't want to wake up to a dead battery so I turned the truck around, hooked a set of heavy cables to it and ran the truck for a hour or so right before bedtime. We didn't use the house lights, instead we used HF led puck lights that last forever on 3 AAAs.
Now I can use that same grp 27 with a 230w portable set up and the only time the battery is not in float is over night and a short time in the morning. After that the panel picks up most of our loads during the day. We also replaced the lights with LEDs.
The difference is wonderful. We no longer conserve and fret over the battery. You would think we have a really long extension cord with us. We set the stat at a comfy level day and night, light the trailer end to end, run the fans, TV, DVVD player, etc. at will. The lowest I see my battery is 12.57v in the early morning if I get up before the sun is and the system starts charging again.
Using just the grp 27 was a test and the panel was flat for it. We have 345ah total as well as a generator to back it up and run the AC if needed. Back up is always a good thing even if it's just a set of jumper cables.
