If the shower is currently TRAPPED above the floor level you just have to get under there and measure what will fit. The sink should be trapped just under the bowl and no problem.
There are a number of companies on line that make tanks and have stock sizes. Many will install the connection for you if given the measurements.
Keep in mind you will have to figure a way to get a vent up to the roof. If the area is small I have used 2 tanks. Cross connecting them at the bottom is not the problem you will have to make a connection across the top to vent the second tank to the main tank. May be a problem depending on the cross bracing of the trailer frame.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles