It sounds like you could get by with just a 12 volt system if you got laptop and cell phone chargers that plug in to a car cigarette lighter.
A pair of 6v golf cart batteries would give lots of capacity, a largish (10 - 20 amp?) charger would let you use shore power when available to recharge the batteries when necessary.
You would not need an inverter (changes 12v DC to 120v AC) or a converter (changes 120v AC to 12v DC). Your old thingy is a converter.
The best way to do this project is to start with an electrical
budget (watts, not dollars). Make a list of each electrical
device, how many watts it consumes and how many hours per day it will operate. Watts is amps times volts. You can do this in amps too if everything is 12 volts. If you have solar
and want to estimate its energy contribution add this to the list as a negative number. Each device on your list will use X amp-hours. Add up the amp hours used in a day. Multiply that number by the number of days you want to go without recharging. Now double the number to get the battery capacity you need.
Let's say you will need 20 amp hours per day and want go without charging for a week. 20 x 7 = 140 a-h. You would need 280 a-h of battery capacity for best battery life. If you got AGM batteries (less maintenance, can be discharged deeper, cost more) you could do the math with a 1.5 multiplier instead of 2. In that case you would need 210 a-h.
PS: Even with all 12v stuff, it would be good to wire in a couple of 120v AC outlets for when you are plugged in to shore power.